also: run-pass option · run pass option
One play that is a run and a pass at the same time, with the quarterback choosing between them off a defender who cannot cover both. Not the same thing as a zone read.
The offensive line blocks a run play, a receiver runs a route, and the quarterback reads one defender to decide whether to hand the ball off or throw it. The defender being read is usually a linebacker or an overhang, and he is being asked to defend a run gap and a throwing window at the same time.
The reason it works is arithmetic rather than deception. If a defender is responsible for both, he can only take one, and the quarterback takes the other. That is the same counting logic as four verticals, applied to one defender instead of a coverage shell.
run-pass optionzone read, play actionThe distinction from a zone read is the one worth carrying: a zone read chooses between two runs, an RPO chooses between a run and a pass. Coaches use "read" for both, which is where the confusion comes from.
The read itself is simple. The rules on ineligible linemen downfield are not, and they differ by level, which is a real constraint on how deep the route can be. That is a rulebook question for your own level rather than a scheme one.