A blitz in football is a defensive play in which more players on the defense rush the offense than there are offensive players guarding the ball. For example, if the quarterback drops back for a pass with six players guarding him and the defense sends seven players to sack the quarterback, that is a blitz.
A blitz is used for primarily two functions: to sack the quarterback before they can throw the ball or guarantee a quick stop on a run play.
A blitz is a very risky play to run. You have more defenders attacking the football, but you also lose out on defenders guarding against the pass. If a blitz does not reach the quarterback fast enough, they will definitely have an open receiver to pass to, which is very costly. Teams also blitz, especially near the end of a game, when they know the offense is going to run the ball.