Nine is greater than five' means that the number of planets is greater than five. It is, of course, true that the number of planets is greater than five, what is not true is that that sentence gives the meaning of the sentence 'nine is greater than five', since 'nine' and 'the number of planets' do not have the same sense, i.e. the same meaning. Davidson's point is not that semantics cannot find a place for meanings, but rather that we can do just as well without them. He hopes that we can give a theory of meaning using only extensions, and construing these extensions as what one might call extensional objects rather than as intensional objects like meanings. By thus employing a sentential connective, we are faced with the problem that we do not have a sentence to go on the left-hand side.
S stands for the name in the metalanguage of a sentence of the object language.
It is, therefore, a name and not itself a sentence. Given a name we can make a sentence by attaching a predicate to it. What we wanted from a theory of meaning was a theory which assigned semantic properties to words, and showed how to derive theorems which give the semantic properties of sentences. If giving a metalanguage sentence which is true if and only if an object language sentence is true is a way of giving the meaning of the object language sentence, then Davidson can take over Tarski's work on truth and use it as the basis for a theory of meaning.