The alley player is a run-support defender responsible for defending the "alley" — the space on the field between the outermost offensive lineman (such as a tight end or offensive tackle) and the wide receivers.
Run Support: When an offense runs the ball outside (such as on a sweep, toss, or off-tackle run), the runner naturally heads toward the open space between the line of scrimmage and the sideline. The alley player must step up into this channel to stop the play.
Tackling in Space: Because the alley is wider and further away from the initial line of scrimmage, the alley player often has to make open-field tackles against speedy running backs or wide receivers.