This is a penalty in the NFL, much to the chagrin of many fans. It is a usually a subjective call made by the officials, and is called when one player stands over the opposition while the opposition is on the ground, spikes the ball aggressively in the direction of the opposition, or performs any other act that could be seen to provoke the other team unnecessarily. This is a serious penalty and results in a 15-yard assessment on the offending team.
This penalty was instituted in the 90's because the League thought players were celebrating too much after touchdowns or big catches, and felt it was distracting to the game. Many fans disagreed, and continue to disagree (unless the call is in favor of their team). The call is most often made on touchdown celebrations and when a player gets in the face of their opponent, or stands over them in a menacing manner.