In fantasy football, usage refers to how involved a player is in his team's offense. The main pieces of usage are targets for receivers and tight ends, carries for running backs, and snap counts for everyone. A player who is on the field a lot and gets the ball often has high usage, which usually leads to more fantasy points.
Usage matters because it is more stable and predictable than raw production. A wide receiver can catch one long touchdown in a game where he saw only three targets, but that kind of scoring is hard to repeat. A receiver who sees ten or more targets every week will keep getting chances to produce even when he has a bad game. For running backs, a heavy workload of carries plus targets in the passing game is a sign the team trusts him.
Fantasy managers track usage to find players who may be undervalued. If a player's numbers look weak but his role is growing, he could be a good trade target or waiver pickup. On the other hand, a player with good stats but a small role may be due for a drop-off. Common stats used to judge usage include target share, snap share, and opportunity share.