Even though we call 365 days a year, the earth does not revolve around the sun in 365 days. Rather, it takes 365 days, 5 hours, 48 minutes, and 46 seconds to do this.
The extra time is made up by adding one extra day to the end of February every four years, EXCEPT in those years which can be divided evenly by 100. Then that extra day is NOT added. However, in years divisible by 400, that extra day IS added.
What that means is simply this, the years 1200, 1600, and 2000 are divisible by 400, so they have the extra day added, making them Leap Years. However, 1300, 1400, 1500, 1700, 1800, and 1900 are not divisible by 400, so they were not Leap Years.