A Leap Year is a year containing one (1) additional day added to keep the calendar year synchronized with the astronomical or seasonal year.  Because seasons and astronomical events do not repeat in a whole number of days, calendars that have the same number of days in each year drift over time with respect to the event that the year is supposed to track.  By inserting an additional day into the year, the drift can be corrected.  This occurs every four (4) years.