An option is a play in which the person who will ultimately carry the ball is not known by the offense or the defense before the ball is snapped. The ball carrier is determined by how the defense reacts.
Certain defenders are intentionally left unblocked. The quarterback “reads” the reactions of these unblocked defenders and either keeps the ball or gives it to another player. When only one defender is being read, the play is an option. When two defenders are read, the play is a triple option.
Leaving a defender unblocked creates a numerical advantage for the offense.