A year containing an extra day. It has 366 days instead of the normal 365 days.
The extra day is added in February, making it 29 days instead of the normal 28 days.
Leap years occur every 4 years. 2024 is a leap year and so is 2028, 2032 and so on.
Except that every 100 years (but not every 400 years) it isn't a leap year!