Full Idea
If it is the case at world W that if event C had not occurred, E would not have occurred either, then the counterfactual means that at the closest worlds to W at which C does not occur, E does not occur either.
Gist of Idea
Causation is when at the closest world without the cause, there is no effect either
Source
David Lewis (On the Plurality of Worlds [1986], 1.6)
Book Reference
Lewis,David: 'On the Plurality of Worlds' [Blackwell 2001], p.78
A Reaction
This is a very Humean account, though updated, which sees nothing more to causation than transworld regularities. To me that is just describing the evidence for causation, not giving an account of it (even if the latter is impossible).