In LibreOffice, years are indicated by four digits, so that the difference between 1/1/99 and 1/1/01 is two years. This Year (two digits) setting allows the user to define the years in which two-digit dates are added to 2000. To illustrate, if you specify a date of 1930 or later, the entry "1/1/20" is recognized as 2020-01-01 instead of 1920-01-01.