It enables use of superuser (root) privileges by select apps. The usual "su" program installed when rooting will prompt for confirmation whenever an app requests superuser privilege, and you can confirm/deny/remember the setting for the app at that time.

Apps need root access to do things that are normally out of reach of an app -- such as a full system backup/restore, using "su" from a terminal, mounting an external USB storage device, etc..

Cheers