A term for when a quarterback throws a short pass to a nearby player after a passing play has had time to develop. Usually, a checkdown is used when the primary receivers are not open, and the quarterback wants to get rid of the ball for some yardage rather than risk a sack. The receiver on a checkdown is usually a running back that comes out of the backfield or swings to the side of the field, and will attempt to gain extra yardage after receiving the pass.
Most pass plays will have a designed checkdown as a safety valve for the quarterback. The checkdown is there after the quarterback has gone through his entire progression and not been able to find an open receiver. Since the pass rush has the advantage the longer a play goes, a quarterback will often take the easy pass and gain some yardage rather than get sacked. The most common checkdown route is having a running back slip out of the backfield and turn around for a reception just a few yards downfield.