the exact meaning is: else
but sometime we use it in some other situations!
example:
دیگه چی؟ what else?
دیگه چیزی نمیخای؟ don't you want anything else?
دیگه دیر شده! it's late by now!
دیگه برو! go now!
دیگه اینجوریه! ok, this is it!
یه کتاب دیگه هم دارم. i have also another book.