>A leap year is a year that has an extra day. This extra day is added at the end of February: February 29th. It's added every four years because Earth does not actually take exactly 365 days to orbit the Sun, but closer to 365 1/4 days. If not for leap years, we'd gradually shift our calendar relative to Earth's orbit.

FYI: In the beginning, the leap day was actually added in between days in February and all patron saints coming after that leap day moved one day over.
Later, the leap day was indeed added at the end of February.