Why do we have leap years?
A year is about 365.24 days, not 365. Adding a day every four years keeps the calendar lined up with the seasons, with a few exceptions to stay accurate.
Going deeper
Without leap days, the calendar would drift by about a day every four years. After a few centuries, summer would arrive in the calendar's autumn.
Adding a day every four years slightly overcorrects, so the Gregorian calendar skips the leap day in century years unless they divide by 400. That's why 1900 wasn't a leap year but 2000 was.