A relay runner is an athlete who participates in a relay race, which is a track and field event where teams of four runners take turns running different legs of the race. The first runner starts the race and passes a baton to the second runner, who then passes it to the third runner, and so on until the fourth runner crosses the finish line. The term 'relay runner' is often used to refer to the first runner in the race, as they are responsible for setting the pace for the rest of the team.