basicaly its doing the date command in cmd and going backwards on it the "length" and pasting character a - b on the file name (may not be understanding or explaing right but thts my take on it (HATE PROGRAMMING)
EDIT: that just gives you day month year or whatever your time settings are however mine gives you year month day ...... (you can change in your regional settings how your time is displayed)
mkdir c:\%date:~-4,4%-%date:~-7,2%-%date:~-10,2%
YEAR - MONTH - DAY