The Earth takes very nearly (365 and 1/4) days to go around the sun.
If our calendar always had 365 days, then the year would end and re-start
too soon, and the beginning of Spring (and every other season) would
eventually drift into the months after March.
If our calendar always had 366 days, then the year would end and re-start
too late, and the beginning of Spring (and every other season) would
eventually drift into the months before March.
We can't make calendars with an extra quarter-day in each year. But we
keep them lined up with the real year by saving up the quarters, and adding
one full day to the calendar every 4 years.