By "a possible world" philosophers mean a total state of affairs in the conception of which no contradiction is involved. Our own world - the real world, the actual one - is just one of many possible worlds. Indeed, it is just one of infinitely many, since for any possible world containing say n atoms there is another logically possible world containing n+1 atoms, and so on ad infinitum. All possible worlds other than the actual world we inhabit are nonactual.