According to Wikipedia: "A ball detent is a simple mechanical arrangement used to hold a moving part in a temporarily fixed position relative to another part.".

In plain english, a ball detent are the things that keep your paintballs from rolling out the barrel of your gun or from double loading paintballs in the breeach, depending on the type of marker you have (open bolt vs closed bolt).

Generally you can see you ball detents by looking down the feed neck of your marker.  They will look like to little nubs or nodules sticking out from the side of the paintball gun.

While not sexiest thing to upgrade on your paintball gun many people feel that upgrading to a better pair of aftermarket ball detents can help increase your balls per second, effeciency and accuaracy.