In your CSV file the date is in the month-day-year order.   The regular expression captures those numbers using parentheses and returns a list of those three numbers in month-day-year order.   The list of numbers is rearranged using a list slice so that the year is moved to the front of the list.   So the original order of month-day-year (0,1,2) is changed to year-month-day (2,0,1).

The date format used has every field as a two digit number so the combination of the year, month and day fields is a string 6 characters long.