For starters, this forum is for international Note3, you have separate at&t note 3 forum, search there as it will have specific rooting info for your model. If you pick up ROMS here you could brick your phone. Supersu is like a supervising program granting permission to access restricted areas and running in administrative mode. If any program requests privileged access, Supersu will ask you if it should grant it. Of course giving privileged root access to virus or some badly behaving program could be very bad idea. I guess you got phone from somebody else, since it's rooted and you don't know about it?
Actually there is a program in Play Store that can check if you have root or not, or you can try to access system files using ES file explorer and see if it works. I'm not sure if having Supersu by itself means you have root ATM, previous owner could have root and maybe he brought it back to stock unrooted leaving Supersu behind?
---------- Post added at 02:08 AM ---------- Previous post was at 01:59 AM ----------
I would strongly suggest for you to do some serious reading in this forums and have full understanding of everything before installing custom ROMS etc. there is very good chance of bricking your device, not covered by warranty, by making even seemingly trivial mistake.