This is a baseball term (though common in other sports as well) which describes an action which disrupts or deters the normal course of play. Interference can be committed by many different parties, and the rules apply differently to each situation depending on the severity of the offense, and the offender. In baseball, offensive and defensive players can interfere with the play by preventing a player from taking a position he has a right to, such as a base runner moving along the base path, or a defender moving towards the ball.
An interference call can also be called if a fan gets in the way of action on the field, or if the field itself somehow interferes with action, perhaps by suddenly interfering in the path of the ball. Some other examples of interference include an offensive player altering the course of a live ball on the field, or a batter blocking the catcher from making a play at the plate. In most cases where an offensive player interferes, an out is called and players are returned to the base they occupied prior to the infraction. In some cases where defensive interference occurs, the offense may be award a base.