It means a player playing the medic class will prefer healing you to other players, and will stick around your position, i.e. stay in your pocket, hence 'pocketing.'

Means you as the medic character stick to healing that one person, becoming their "pocket medic." Commonly people pocket medic heavys, the biggest characters who have a large spray weapon and high max HP.

I guess he'll pick a medic class and will heal you constantly for a small compensation in bitcoins.