clonecd sub file when making cd image

hi guys.i'm using clonecd at the moment to make images of ps1 games to play on my psp and when finished there is an img and a sub file.i dont know what to do with this file or if it is even important or not.please tell me what to do

you can use this video tutorial.

http://www.gametrailers.com/player/user ... 40701.html