It means Presidential Niggas Or Die. This usually refers to a group of cool guys with alot of swagger and usually have alot of girlfriends who they sometimes refer to as bitches. It is usually written as PNOD! and maybe seen on many sufeaces like walls, desks e.t.c.
Girl: Who are those handsome guys?
Guy: Oh, thats PNOD!
Girl: Hmmm ;)