A rundown in baseball is a situation where a base-runner is in a running lane in between two bases and two fielders trying to chase them down for a tag out. It is also known as a pickle from the base-runner's perspective.
A rundown, sometimes called a pickle, is when a base runner is stranded between two unoccupied bases, with a fielder on either side of him having the baseball. This sometimes happens when a batter-runner overestimates which base he can get to safely before the infielders get the baseball. As the runner tries to reach one of the bases he is stuck between, two infielders will surround him, and toss the baseball to each other depending on which direction the runner is going. Often, the runner will switch directions with the baseball, or try to run past the fielder to avoid being tagged out.