In cricket, the boundary is the perimeter of a playing field. It is also the term given to a scoring shot where the ball is hit to, or beyond, that perimeter, which generally earns four or six runs for the batting team.
Briefly, if the ball is struck by the batter and rolls or bounces over the boundary (or just touches it) it is known as a "four", and scores four runs, whereas if it flies over (or touches) the boundary, without touching the ground before that, it is called a "six", and scores six runs. There are rules covering every possible situation, including the fairly common one when a fielder is in the air beyond the boundary when he or she catches or strikes the ball with his or her hand or another part of the body.