In gaming, a leech is a person who takes without giving back. In other words, someone who profits massively from others without contributing anything to their success.
In gaming, for example, these are situations in which a player benefits massively from the loot of his team, although he himself does not search properly. Or, they get experience points and items from their own team's fights, but keep themselves out of conflicts and battles.
So you benefit greatly from your team's efforts while sitting back yourself. It's not surprising, then, that leeching is usually viewed rather negatively and should be understood more as a nitpick or slight insult.