S-off is a setting in the root that lets you write to and modify system files while the phone is running. If you're not rooted, it means nothing to you, as you can't access the setting. I'm not yet rooted and my knowledge of such things comes from a close friend who is and who hacks his phone regularly. Basically, s-off renders the phone wide open to all modification of software. It has great potential reward for those with the knowledge of what they're doing and the greatest risk of a bricked phone to the uninitiated.