What is the first animal to come to your mind when I ask this question? I bet at least one animal comes to mind. But why did the animal you chose come to mind? Why did, say, a blue spotted salamander or box turtle or naked mole rat come to mind when I asked the question? You might reply that it is because it was the animal you chose to think about, using your own free will. But was it, though?
A possible world is a way in which it is logically possible (does not result in contradiction) for reality to exist. There is a possible world in which everything is exactly the same, but it is 1 degree warmer than it is now; there is a possible world in which COVID-19 never existed; there is a possible world in which everything is exactly the same, but there is +1 more person existing right now. What is the nature of these possible worlds, and specifically, what is the nature of my relationship to myself in these possible worlds?