what's the meaning of piss off? and what about piss somebody off?

Question

Answer

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

Answer

piss off = go away = beat it = get lost

piss someone off = it means to make that person super unhappy :P

Answer

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.