This baseball term refers to action taken by a fan that causes disruption of the game, usually involving contact with a ball in play or with a baseball player. More specifically, a case of fan interference with respect to the field of play occurs when the fan crosses over the invisible boundary separating the spectator space from the playing field.
When fan interference happens, the umpire making the call on the play decides how the play would have concluded, calling outs and awarding bases as needed. Should a fan catch a ball or interfere with a catch when on his/her side of the line (i.e. out of play), there is usually no ruling of interference. A fan will generally be ejected from the baseball stadium if it is determined that he or she has interfered with the normal course of the game.