Unbirth, in short, is precisely what it sounds like. Birth in reverse, or basically, taking an already-born person/entity and letting them back into the womb. Obviously, this isn't possible in real life, so it's basically constrained to fantasy and roleplay. As yet, even Second Life doesn't make this possible.
Unbirth is one of those things that just doesn't make any sense to most people. To be frank it didn't really make sense to me, either, the first time I read about it. I mean, how would it even work? But then I spent more time hanging around in fora where it was an important secondary kink. Secondary, that is, to maiesiophilia, and usually in a furry context. After a while of this, I finally zenned why it was that people liked it. In short, it's ultimate intimacy combined with making your partner pregnant or at least virtually so, all without the weirdness/inconvenience of virtual kids. Now, I know that all sounds pretty weird, and maybe a little squicky, but hey, it's all fantasy anyway.
The question that comes up, time and again, when unbirth is mentioned, is this: is it a form of vore? Heck, the phrase "vaginal vore" pops up once in a great while anyway, and what is that, if not unbirth? I'm not sure whether or not there is a consensus on this, but most who have this kink agree that it's rather like vore in some ways, but is very much distinct. In particular, it has rather fewer of the domination and submission overtones - who's doing the unbirthing is very much limited to who's equipped to do so, regardless of who's dominant.
The other perennial question is, what happens after you get unbirthed, and how do you get out? There's as many answers to these as there are folks with the kink. Some say you revert to infancy. Others, you retain your adult body and mind and are just compressed or scaled down to fit - it's fantasy, after all, who says matter isn't mutable - or that the space you're in is magically big enough for you without scaling. After all, why couldn't your love's womb secretly be a TARDIS? (That's the magic of fantasy, that it is!) And getting back out ranges from simply wishing yourself out, to getting back out the way you did the first time - by being born. This can be as easy - or as hard - as the participants want.
So yeah, it's a bunch of fantasy. Kinky, confusing, consensual and harmless fantasy. It sure is weird, but hey, thousands of furs can't be too far wrong!