A complete revolution around the Sun takes about 365 days and 6 hours. That's why our Gregorian calendar has a leap day.
There is a leap day every 4 years because Earth completes one revolution around the sun in around 365 days and 6 hours. Over a period of 4 years, the extra 6 hours from every year get added up to become an entire day (6*4=24 hours). Hence, we have a leap day/leap year every 4 years.
The month of February is a wee bit different from other months in the sense that every 4 years, out of nowhere, a 29th day is added to it, and the month of February becomes one day longer than usual!
A leap year consists of 366 days as opposed to a 'regular' year that consists of 365 days. Every 4 years, a year comes along that has one more day to it, meaning 366 days in total. Those years with 366 days are leap years. More specifically, the year in which the month of February has 29 days (instead of the usual 28 days) is deemed a leap year. For example, the year 2016 was a leap year.