For a drummer to hear "you have a good pocket" or "you have a deep pocket" is probably one of the highest compliments you can receive.

To "play in the pocket" means that you have a good feel, and that your groove is right on the money. You are laying it down in such a way that you make the music feel good, as well as the making the other musicians playing with you ... feel good. You are playing with conviction and letting the tempo move all around the place. You make it easy for other musicians to play with you.

This is just some, but not all, of the meanings this would have if someone said you are a good pocket player.