A normal year has 365 days. However, every four years there is a leap year with 366 days. This is due to the fact that the earth does not need exactly 365 days to travel once around the sun, but about a quarter day more. Every four years a whole day comes together in such a way.
So that our calendar is nevertheless "correct", an additional day is inserted every four years. This is the 29th of February, which does not exist otherwise. People born on February 29th therefore only have a birthday every four years. In the other years, however, most celebrate their birthday on March 1.
The secret of leap years was discovered by the ancient Romans. Their calendar originally had only 355 days, so it was ten days too short. This led to the fact that with the time the seasons got mixed up. That's why Julius Caesar introduced a new calendar in 45 BC. Since then, the year has 365 days, and every four years there is a leap year with 366 days.