Joseph-
A solar year is approximately 365.2422 days long. The Julian Calendar rounded the value to 365.25 and had a leap year every four years (1/4 = .25). Pope Gregory the 10th, 11th, 12th, 13th or 14th(I forget what number) realized the mistake and tried to fix it.
As of Thursday, October 4th 1582 the Julian Calendar ceased to exist and the next day the Gregorian Calendar took over with the date being Friday, October 15th 1582. (The skipped days compensated for the slight decimal error over close to 16 centuries.)
The new calendar calculates the fractional part of the day as .2425 which is equal to 97/400. Therefore 97 out of 400 years are leap years.
In general a year that is divisible by 4 is a leap year. The exception to that rule is if the year is divisible by 100. The exception to that rule is if the year is divisible by 400.
1892, 1896, 1904 and 2000 were leap years.
1700, 1800 and 1900 were not. 2100 will be the next multiple of 4 that’s not a leap year.