The WR Screen is a play that utilizes blocking by the inside Receivers to enable a running lane for the outside Receiver. The Wide Receiver running the screen route needs to push up field hard for two steps, and the quickly turn around for the ball. Timing is critical on the throw.
The inside Receiver needs to explode of the line of scrimmage directly at the guy who is guarding the outside Receiver. Once that defender turns around, quickly get in his way for the block, and the Receiver catching the screen will be off to the races.