The custom which eventually became part of the Constitution as the 22nd amendment is that no president can be elected for a third term.
This means that an American politician can be chosen only twice to serve as president for the duration of 8 years in total, if chosen twice, or 4 years, if chosen once. This was the custom since the creation of America, but Franklin D Roosevelt broke this rule and served for 4 terms, after which the 22nd amendment was created.