A leap year has 366 days. This is one extra day compared to a common year, which has 365 days. The extra day is added to February, making it 29 days long instead of the usual 28.
Leap years are necessary because the Earth takes approximately 365.25 days to orbit the Sun, not an exact 365. Adding an extra day every four years helps to synchronize our calendar year with the astronomical year. Without leap years, the calendar would gradually drift, causing seasons to occur at different times of the year over many centuries.