A no-ball is an illegal delivery. The most common kind is a front-foot no-ball, when the bowler lands without any part of the front foot behind the popping crease. Deliveries that are too high on the full, or bowled with an illegal action, are also no-balls.
A no-ball costs one run, the delivery does not count as one of the over’s six balls, and the batter cannot be out in most ways from it.
The free hit
In limited-overs international cricket, the next delivery after any no-ball is a free hit. The batter can only be dismissed by being run out, hitting the ball twice or obstructing the field, so they can swing freely without fear of being caught or bowled.
The fielding side is also restricted: it cannot change its field for the free hit unless the batters have swapped ends.
Who calls it?
In most international matches, the TV umpire now checks every front-foot no-ball, so the on-field umpire can concentrate on the rest of the game.