Every four years an extra day is added to the calendar, making
the length of the year 366 days, instead of the normal 365.

Why on earth does that need to happen?

From Kids Know It Network- The calendar is supposed to match the solar year, in other words, the length of time it takes for Earth to orbit the Sun once. But things aren’t quite that simple. It actually takes Earth 365 days, 5 hours, 48 minutes and 46 seconds to complete its orbit (about 365 1/4 days). Those extra hours gradually add up so that after four years the calendar is out of step by about one day. Adding a day every four years allows the calendar to match up to the solar year again.