Leap Day is typically every 4 years and is the day of the year where we adjust for the extra approximate 1/4 day in the earths 365.242190 rotation around the sun. This is about 6 hours more time per year.
Leap Day is every 4 years except years ending with double zero digits that are not divisible by 400. It may seem confusing, but because the rotation is not on an exact 365.25 rotation, we have to adjust for the difference every so often. So the years that are divisible by 100 and not 400, skip a leap day. The next time this will occur is in the year 2100. From there, 2200 and 2300 will also skip a leap day, but 2400 will keep the leap day.