A leap year is a year in which an extra day is added to the Gregorian calendar, which is used by most of the world. While an ordinary year has 365 days, a leap year has 366 days. The extra day, February 29, is added to the month of February. In an ordinary year, February has 28 days, but in a leap year, it has 29 days. The extra day, called a leap day, is the same day of the week as the first day of the month, February 1. Also, in a leap year, the months of January, April, and July all start on the same day of the week.
A leap year comes once every four years. Because of this, a leap year can always be evenly divided by four. For example, 2020 (this year) is a leap year. But a year is a common year if it can be evenly divided by 100 but not by 400. This is why 1600, 2000, and 2400 are leap years, but 1700, 1800, and 1900 are common years.
We have leap years because instead of 365 days, the Earth really takes a few minutes less than 365-1/4 days (365.24219) to go completely around the Sun. Without leap years, the seasons would start one day earlier on the calendar every four years. After 360 years, spring in the Northern Hemisphere and autumn in the Southern Hemisphere (which usually begins on March 21) would begin on December 21 (which is when winter in the Northern Hemisphere and summer in the Southern Hemisphere presently begins).
A number of countries use a lunar calendar (based on the Moon, instead of the Sun, like our solar calendar is). They have leap years when they add an extra lunar month. Different calendars add the extra month in different ways. So a year which has 366 days instead of 365 days where the month of February has 29 days is called a leap year.