Leap Day is confusing, but this scientific video will help you understand it. Basically, what it comes down to is that Earth, like the jerk it is, refuses to rotate 365 times during its orbit of the sun, and instead revolves 365.24219 times. This stubbornness puts our calendar year off by about six hours.
So Julius Caesar came up with a solution back in 45 B.C.: every four years, we have Leap Day, February 29. The other three years, we just ignore "that extra quarter of a day." So if we didn't add an extra day every four years (Leap Day), because of the way Earth tilts (wow, get it together already, Earth) our calendar would eventually become out of whack with the seasons—Christmas would be accompanied by summer weather.
Leap Year actually overcorrects the calendar just a smidge, so every hundred years, we skip Leap Year, and to make up for that, every 400 years, we skip skipping Leap Year. Oops, it just got confusing again.