When we talk about possible worlds,we don´t think about them in ontologycal way(existence of ather world then ours).Possible worlds can be explained as all possible situations we can think of.For example,we will say that something is necessery if we can not think of any situation in wich that not exist.In short we will say it exists in all possible worlds.it is hard for me to explain it,because of my bad english and because it is complex field(modal logic).