A runner is a teammate who does the sprinting for a batter who is too injured to run but still healthy enough to hit the ball. The runner must wear the same protective gear and carry a bat while standing at a safe distance near the umpire. Example: The captain acted as a runner for the injured star, who could barely walk but wanted to finish his century.
Pro-Tip: This rule has been removed from most international matches to prevent teams from gaining an unfair advantage, but it is still seen in some local and amateur leagues.