A leap year is a year with an extra day, February 29. It exists because Earth's trip around the Sun takes about 365.24 days, not exactly 365 days.
Why the extra day is needed
If calendars ignored the extra fraction of a day, seasons would slowly drift away from the calendar dates. Adding leap days keeps the calendar closer to the solar year.
The common leap year rule
In the Gregorian calendar, most years divisible by 4 are leap years. However, years divisible by 100 are not leap years unless they are also divisible by 400.
Leap years are a calendar correction, much like time zones are a practical way to organize time around the world.