Physicalism is the metaphysical thesis that everything is physical. That is to say, the nature of the actual world (i.e. the universe and everything in it) is such that it is fundamentally physical.
Now, some argue that the possibility of zombies means that physicalism is false.
But, even if it is granted that zombies are possible, how does their possibility pose a threat to a thesis about the actual world? After all, all that follows from the possibility of zombies is that there is a possible world in which there are zombies. But that is consistent with the thesis that the actual world is physical. So why should physicalists worry about zombies?