The leap days, added to the end of February every four years, keep our calendars in sync with Earth's orbit. Our standard calendar year has 365 days - but Earth actually takes 365,256 days to orbit the sun.
If you add February 29th to the calendar every 4 years, you make up for that quarter day. In 2020, two scientists proposed abolishing leap years and creating a new annual schedule: one that is 364 days long, with the year always starting on a Monday and most months being 30 days long.