It stands for
Back
Up
Iron
Sight. The idea behind it is that the optic is powered by batteries, and is electronic, which makes it the weakest link. Having a BUIS allows you to flip upan iron sight, and continue the gunfight.
I've had an optic go down in the real world, and I can tell you that with solid training a BUIS only takes a second or so to deploy and get back into the game.
BUIS range in price from $65 or so, to $150. There are a few good lower priced BUIS on the market right now. When selecting a BUIS, you need to figure if you want one that folds flat, or one that stays up all the time. If you go with a folder, make sure it isn't one that requires you to grab it by the windage knob to raise. Adrenaline in combat will cause you to get an unwanted few cranks in while you are raising it. Its a topic which isn't brought up often, but it is one that I'm a big believer in.
If you are looking at it only as a range toy, you can go with just about any of them, and they will all work very well.