From BR Bullpen
The warning track is a wide path at the base of the outfield fence made of a different material from the playing surface of the field of play. Its purpose is to tell the outfielders that they are approaching the fence, in order for them to avoid contact at full speed with the obstacle. The warning track will often be made of sand on natural playing surfaces or rubber on astroturf or other artificial surfaces. The outfielder will know he has reached the warning track without having to take his eyes off the ball.
Warning track power is a derogatory expression that refers to a hitter who hits long fly balls that are caught just in front of the outfield fence. These may look spectacular, but are nearly useless in terms of putting runs on the scoreboard (except from netting the occasional sacrifice fly).