A free safety in football is a defensive back who can roam any part of the field, hence the term “free”. This is different from strong safety, in which the SS lines up on the same side as the tight end.
Famous free safeties include Ed Reed, Ronnie Lott, and Steve Atwater.
In Seahawks Football 101, former Seahawk Brian Russell illustrates the free safety’s job of roaming the field as the QB is ready to snap the ball.
As you can see, Russell actually went too far back on the Cover-2 zone shell and landed himself in the Cretaceous period. The quarterback threw the ball to an open receiver in the middle of the field for a first down.
Fun note: When I was editing this photo I kept giving off the impression a chunk of Russell’s right leg was bitten off. After careful erasing procedures I think I’ve corrected that. I’ve also given the impression that Brian can walk on water. That’s interesting.