If used as a command, "Piss off!" it means "go away!" To "piss someone off" means to "make someone angry." If someone is pissed off, it just means really mad or angry

piss off = go away = beat it = get lost piss someone off = it means to make that person super unhappy :P

As a command it is rude. As a state of being, it is only slightly less rude. There are other, more acceptable English terms to learn.