Typically an outside linebacker, defensive end, or safety — against the run game.
The primary job of the force player is to:
Contain the ball carrier: Prevent the running back or quarterback from getting outside of the defense and turning the play up the sideline for a big gain.
Turn the play back inside: Force the runner to redirect their path toward the pursuit of the inside linebackers and defensive linemen (often referred to as "spill" or funneling the play).
Depending on the scheme, the force player is responsible for maintaining outside leverage on the outermost blocker (like a tight end or wide receiver) and either meeting the runner head-on to make a tackle or squeezing the gap so the runner has nowhere to go.