A play fake in Football is another term for a play-action play. A play-action play is a play in which a fake handoff occurs, or any sort of fake occurs. In most cases, a play-action play is a passing play in which the quarterback fakes handing the ball to the running back, then passes the ball downfield.
Play fakes aren't limited to play-actions. Play fakes can be fake tosses to the running back, decoy punt and kick returners, and fake ballcarriers.