What is the meaning of Drop_back?

Of a quarterback or other player in the backfield, to take a number of steps back from the line of scrimmage immediately after the snap or hike of the ball, to avoid defenders.

To move to a lower (or slower) position in a race or competition; to slow down so as to meet up with someone behind oneself

Used other than figuratively or idiomatically: see drop, back.

The act by a player in the backfield of dropping back from the line of scrimmage after the hike of the ball.

Source: wiktionary.org