Our calendar year is based (somewhat loosely) on the amount of time Earth takes to complete one orbit around the Sun. This time period is known as a solar year and is about 365 days and 6 hours long. Our calendar year has only 365 days.
This anomaly causes an accumulation of (almost) 1 day every four years and, unless corrected, will cause an ever increasing gap between calendar dates and astronomical events. To avoid that, we add 1 day to the month of February every 4th year and call that year as the Leap Year. One of the possible reasons for adding the extra day to February is because it is the shortest month of the year, with only 28 days (all other months have 30 or 31 days).
Hope this helps.