Full Idea
The central idea in possible worlds semantics is that the modal operators are treated as quantifiers.
Clarification
'Semantics' concerns meaning; modals are necessity/possibility; quantifiers tell you what exists
Gist of Idea
In possible worlds semantics the modal operators are treated as quantifiers
Source
Joseph Melia (Modality [2003], Ch.2)
Book Reference
Melia,Joseph: 'Modality' [Acumen 2003], p.49
A Reaction
It seems an essential requirement of metaphysics that an account be given of possibility and necessity, and it is also a good dream to keep the ontology simple. Commitment to possible worlds is the bizarre outcome of this dream.