The root user-account on linux systems has rights to do anything to the system (for example, something dumb like deleting all files).

If you're in a terminal using a gainroot-type-program like rootsh, becomeroot or easyroot then becoming root switches you from being the regular "user" to being the "root" user.
It's temporary, so it lasts until you switch back to being the regular user (for example, with 'exit', or by closing the terminal app).