An offensive player who is not being guarded by a defensive player during a play. Open receivers tend to be downfield during a pass play and have found an open space on the field where a quarterback can deliver a pass to them. Open receivers are far more likely to catch a pass from a quarterback, as well as make big plays on offense.
A quarterback is likely to try to find an open receiver downfield for a number of reasons. An open receiver means a high percentage throw, since there is not a defender able to make a play on the ball. It also means the receiver may have a chance to run after the catch and make more yards on the play. A receiver can become open because of a well-run route, a breakdown in coverage by the defense, or a play that continues for a long time since it is tougher on the defense to guard a receiver the longer a play continues. Most pass routes are designed to create an open receiver.