To bunt is to hit a baseball in a deliberately light way, without swinging. Some players bunt so that another runner can get from second to third base.
The bunt is a rarely-used technique in baseball. When a batter bunts, the ball doesn't travel far — there's basically no chance that a bunt will result in a home run, for example. It forces the fielders to run forward, which can sometimes be an advantage for the team that's at bat. The earliest meaning of bunt, "to strike with the head or horns" is probably an early variation on butt, with the baseball meaning arising in the 19th century.