A touchdown vulture is a backup who swoops in to steal scoring chances from a more talented teammate. The classic example is a big, bruising running back who enters the game when his team is inside the five-yard line. The starter does the hard work to move the ball down the field, but the vulture gets the easy carry across the goal line and walks away with the touchdown.
Vultures matter in fantasy because touchdowns are worth so many points. If you own the starter, a vulture cuts into your scoring every week and can be frustrating to watch. If you own the vulture, though, you have a cheap source of touchdowns, and some fantasy managers deliberately draft these players late for exactly that reason.
Vultures are not always running backs. A backup quarterback might steal a rushing touchdown with a sneak, or a tight end might catch a pass at the goal line that a receiver would normally get. Whenever a part-time player takes scoring chances away from a starter, fantasy managers call it vulturing.