This is a phrase used in football which describes the action of one or more players jumping on top of a player or group of players after a tackle has been made. This is illegal at most every level of competition, with only the specifics as to what constitutes "piling on" and what the penalty is. This action could also be considered a late hit. In the National Football League, the offending team would be charged with a fifteen yard penalty.
This action is illegal because it slows down the game, has the potential to injure a player, and is an unnecessary action. Although players wear lots of padding during a game, piling on can cause injuries because of the sheer weight of the players on the pile. As soon as the referee blows their whistle, the play is dead and no hits are allowed. If a defensive player is already in motion to make a tackle, they must pull up and prevent the hit, or risk a penalty.