What is the Flat in Football?
The "flat" in football refers to the passing zone located to the outside of the weak tackle or tight end. It extends to the sideline, and is about eight yards from the line of scrimmage. It is also referred to as the bench route, as the player is running towards the benches. This route is run by the running back and fullbacks from the back field, they immediately run towards the sideline, into the flat, and downfield. As the receiver runs they're constantly looking behind their shoulders for the incoming ball. While effective, the flat route isn't run more than 4 or 5 yards as it's a somewhat lateral path of running.
It's important to stay inbounds when running the flat route, especially because it is so close to the sideline. If the ball hasn't been thrown, and the receiver is 2 yards from the sideline, the receiver should stop running downfield and prepare by getting into an athletic position.