A fast break in basketball is a play in which the defense transitions quickly from defense to offense by making quick passes and running the court, leading to an easy score. A fast break starts with a mistake by the opponent, like a turnover or a missed shot. However, for a fast break to happen, the other team must take advantage of their opponent's mistake. As soon as they get the ball, the team must quickly pass the ball, and all the players of the attacking team must run to the frontcourt as to give passing options to their teammates.
The goal of running and passing fast is to catch the opponent off guard and disorganized, leading to an easy and unguarded basket, preferably a layup or a dunk. Fast breaks are widely practiced by teams of all levels and are a very important part of a basketball game. An example of that is that you may hear analysts and specialists talking about a team's fast break points or fast break efficiency, which are both statistics used to compare teams.