If you're reading this, then more likely than not you've gotten yourself jacked into a Digitial Web search-spider and plugged in any one of the following terms: "Umbra", "High Umbra", "High Astral", "Astral Courts", "Angel". I've set the metadata on this file such that it will fly right to the top of pretty much any search, which is the entire damn point. If you wan to travel to the Courts of the High Astral Realms...
Don't. I'm serious. There are better ways to get yourself fragged, shot, disassembled, possessed, or otherwise have your day ruined. Forget about whatever valid reason you have for wanting to go there - chances are whatever you're looking for you're not going to find. That's the nature of the place.
You, of course, are more likely a 20-something mage with a little knowledge and a whole lot of attitude, so more likely than not you're planning on going anyway and screw what some guy out on the 'Net has to say. Fair cop - I'd probably do the same. So here is a brief rundown on what to expect when you try and make your way across the Pearl-Bright Ocean to the far side of reality...
OK, quick cosmology rundown. The Umbra is the spiritual reflection of our physical world, but just as our world has many facets, so does the Umbra. Modern cosmology breaks the whole thing down into three parts: the Low Umbra (aka the Shadowlands, aka the Land of the Dead), the Middle Umbra (aka the Velvet Shadow, aka Werewolf Country), and the High Umbra (aka the Astral Courts). Let's disregard the other two for a moment and just focus on the Astral Realm, because that is where you want to go.
To sum up, the Astral Realm is the Realm of Abstract Embodiment. Concepts can take physical form here, and the symbolic is real. Let me say that again: in the Astral Courts, symbolism is real. By extension, all reality is symbolic. This is something most people don't grok the first couple of times they read that sentence. Mages are, of course, heavily into symbolism. Reality is too much to swallow all at once, so instead we deal with the particular fragments of the Universe that we're interested in by using symbols. A bowl of water to represent the ocean, a particular equation to represent the human mind - it's the way we work. We understand that there is something deeper at work behind our symbols, but we also understand the different between a symbol for something and the thing itself.
In the Astral, that difference ceases to exist. That bowl of water is the ocean, and the equation is a person. Now apply this to everything - from the way you talk right down to Nike logo on your sneakers. This is the first big problem with the Astral Courts - human beings aren't used to being symbolicly potent every second of every day. It takes a lot of self-control to control your voice and mannerisms to the degree necessary to keep you stable in the Astral - more often than not you'll simply do a bunch of contradictory things that the Realm will resolve by kicking you out.
Then there is the matter of Threshold Guardians. There are lots of them, depending upon the path you take and the time of year and your birth sign and a whole lot of other mystical nonsense. There are plenty of other books you can read about various Guardians on the Threshold, I'm not going to go into detail here. Suffice it to say that if you do a whole lot of research, walk just the right path and are very, very lucky you'll make it past the Threshold to the Astral Courts themselves.
OK, so let's assume that for better or for worse you've actually made it past the Threshold and into the Astral Courts. I'll talk a little more about the nature of the Courts themselves now. Remember that the Astral Realm is the place of Abstract Embodiment - concepts can (and will) take physical form. The stronger the concept, the stronger the form. The more complex the concept...you get my drift. You can find almost anything here, if you know where to look. Most concepts just operate on their own, running around intereacting with others and going about the day-to-day business of being, say, Temperance or Fiscal Responsibility or what-have-you.
The more powerful ones, the Celestines and Incarna, have developed retinues - servants of their own. Perhaps other concepts pay fealty to them, or they've acquired power by devouring other beings and assimilating aspects of their nature. These are the ones to watch out for, because they've interacted more with the material plane and will be able to spot you for what you are. They get out a lot more, but to explain how that works I'll need to explain angels.
Some of them do have the traditional uniform - white wings, halos, harps, the whole nine yards. More often you'll see some kind of variation, though. An angel of war with steel blades for feathers, an angel of hope with shimmering wings of gossamer light. The most important thing to remember about angels is that they are constructed beings - servitors crafted by their masters to fulfill a particular function.
Presumambly, by the time you become so important and powerful to be an Incarna, you no longer have the time or ability to go around and attend to all the thigns that got you where you are today. So you make angels (this is a generic term - they go by lots of titles) to do things you can. Often this involves travelling to various Umbral places. Sometimes it involves incarnating on the physical plane - a job generally regarded as reserved for the lowliest servants. Angels claim this is because the material world is "beneath" them, but it's really a power-level thing. Angels are crafted out of the stuff of the Astral, and as such they are at home in realms of similarly, um, ephemeral reality. Earth is very solid, and a powerful angel has trouble fitting all of that power into a tangible form in order to do his master's bidding. Over periods of time, the angel will begin to degrade as reality (and Paradox) takes its toll. As a result, most high-ranking angels tend to affairs in the realms of spirit, and leave Earthly affairs to peons. I understand that some powerful servants have tricks that allow them to function more easily on the material plane, although they still pay a high price when they manifest their true nature.
The other curious thing about angels is their relationship to free will. Outside of some monotheistic navel-gazers, most of us more or less accept the fact that we have free will to do whatever we damn well please. Actions may have various consequences, but we are always free to act in almost limitless ways if we are willing to pay the price and reap the benefits. Angels are constructed beings, and so by design their "minds" totally reflect the being that created them. But because the greater powers in the Courts are, themselves, somewhat single-minded, this can create problems. Things are somewhat more unpredictable outside the Astral Courts, and a servant who can't cope is no good whatsoever.
As a result, some angels are imbued with a certain amount of free will - independance from their creator's mindset, that allows them to cope with circumstance and deal with the unexpected. The danger, however, is the Lucifer problem - the more free will an angel has, the more likely it is to go AWOL and tell it's creator all the various ways that he/she/it can Get Bent. What you usually get, then, is a trade-off: angels with a lot of free will tend to be not as powerful, to reduce the risk of some psycho being running amok and doing damage to his creator's agenda. Higher-powered angels usually have less free will, because they are a lot less likely to go rogue.
Well, that's the basis of it. Hopefully this long diatribe has made you reconsider the consequences of heading into the High Umbra - if you are pigheaded enought to remain undeterred, that's your own problem. All I can tell you is to do a lot of research so you know what you're getting into, and be ready to bail if things get too rough. Oh, and if you ever encounter Overconfidence, tell him he still owes me 20 quid for that bar bet...