A leap year is a year with 366 days instead of the usual 365 days. This is done to keep the calendar in sync with the Earth’s orbit around the sun. The Earth takes approximately 365.242 days to orbit the sun, so every four years, an extra day is added to the calendar to make up for the difference.
There are specific rules to determine whether a year is a leap year or not. According to the Gregorian calendar, which is the most widely used calendar today, a year is a leap year if it is divisible by 400 or if it is divisible by 4 and not divisible by 100.