Full Idea
To say than x has a property in a possible world is simply to say that x would have had the property if that world had been actual.
Gist of Idea
Asserting a possible property is to say it would have had the property if that world had been actual
Source
Alvin Plantinga (Transworld Identity or worldbound Individuals? [1973], I)
Book Reference
'The Possible and the Actual', ed/tr. Loux,Michael J. [Cornell 1979], p.155
A Reaction
Plantinga tries to defuse all the problems with identity across possible worlds, by hanging on to subjunctive verbs and modal modifiers. The point, though, was to explain these, or at least to try to give their logical form.