The BMU is a relay of sorts. It does three things:
1) Tests the brake switches. When you start the bike up, it illuminates the red "triangle light" in the center of the instrument cluster. Once you've hit both brakes, it should go out. If it doesn't then one of your brake switches is probably bad.
2) Tests the tail light.
3) Tests the brake light.
It's a safety device so you're not running around with a bad brake switch or lights that are out.
Note that the "triangle light" may flicker or go solid while you're riding. Sometimes this is caused by a tail or brake light bulb getting old even though it lights up as it should. Try cleaning the contacts in the socket or replacing the bulb.