"Most often used in conjunction with a class type, e.g. "Pocket Medic" or "Pocket Pyro", the term describes a player who is focusing support to a single other teammate. The term, often used in the context of "Pocketing", references the idea of taking the supporting player, putting them in a pocket, and carrying them around. Pocketing is often used as a negative term in public play because of the Pocket's tendency to ignore other players that may need healing, while it is considered a normal part of competitive play because of the smaller team sizes and the strategies that involve it."Err, I'm still learning the terminology, what's pocketing?
http://wiki.teamfortress.com/wiki/Player_terms