Full Idea
Possible worlds can be thought of as indices for models of the language in question, or as concrete realities (David Lewis), or as abstract ways the world might be (Robert Stalnaker), or in various other ways.
Clarification
'Indices' are (presumably) theoretical marker posts
Gist of Idea
Possible worlds are indices for a language, or concrete realities, or abstract possibilities
Source
John Perry (Knowledge, Possibility and Consciousness [2001], §8.1)
Book Reference
Perry,John: 'Knowledge, Possibility and Consciousness' [MIT 2001], p.170
A Reaction
I strongly favour the Stalnaker route here. Reducing great metaphysics to mere language I find abhorrent, and I suspect that Lewis was trapped by his commitment to strong empiricism. We must embrace abstractions into our ontology.