Basically, "step off" is a form of brush-off, and/or an order to another individual to get out of your sight. You don't want them talking to you, or even near you. You want nothing to do with them.
"Step off. You're not welcome here."
Translation for the layperson: it is basically a clean version of "fuck off," sans profanity. It is unlikely that one would say this to another without a justified/specific reason to be pissed at them. Basically, one is ordering the other to not talk to them, or even come near them. Preferably get out of their sight altogether.
"Better step off winky. I know all about what you did."