In leap year there is an additional day added to the calendar to keep the calendar year synchronized with astronomical year.
The reason for adding an extra day to the calendar is that the Earth takes appr 365.2422 days to complete one orbit around the sun.
Therefore, a leap year, which has 366 days instead of the usual 365, is needed every four years make up for the additional time.
Every year that is divisible by 4 is a leap year. A year that is divisible by 100 is not leap year unless it is also divisible by 400.
For example, the year 1900 was not a leap year, but the year 2000 was a leap year because is divisible by 400.
That is why next leap year after 2021 will be 2024 and after it 2028, 2032 and so on.
A leap year has an extra day in February because February is the shortest month of the year and has 29 days in a leap year.