The space directly behind an opponent, where the player on the ball simply cannot reach you. It travels with the defender — standing 'free' counts for nothing there.
You are unmarked, you raise your hand, and the ball still never comes — nine times out of ten you are stuck in the cover shadow. A defender pressing the ball screens more than himself: his body blocks the whole passing lane behind him. So the first question when getting free is never 'am I unmarked?' but 'can the player on the ball actually see me?'. The classic error is standing still, waiting for the ball instead of opening the lane yourself. Two quick, short steps sideways are usually all it takes. The catch: the shadow belongs to the defender, not to the pitch. When the ball moves, the shadow swings with it and the position that was good a second ago is shut again. Getting free is therefore not one sprint but constant small adjustments — and that is what separates players who see a lot of the ball from those who feel ignored.