In Dungeons & Dragons' third edition Player's Handbook, Combat Reflexes is described as a feat (special ability) that your player character (or "PC") can acquire. From the Player's Handbook (p.80):

"You can respond quickly and repeatedly to opponents who let their defenses down.

Seriously enough, I believe this feat is a waste of time -- why are you letting enemies close enough to you as to have to make opportunity attacks anyway? If you play a class that primarily acts on the front line (such as a barbarian), this feat could be useful if you plan on flinging the guy into a horde of baddies. Worthless for people who aren't keen on dying.