Where's the notion of interpretation (model) originally introduced? I find it used in Skolem's paper "Logico-combinatorial investigations in the satisfiability or provability of mathematical propositions" (1920). However, I do not find any explicit reference to semantical ideas in Frege's Begriffsschrift. Where did the formal idea of semantics come up? What's the relation of this to Tarski's theory of truth? I really have a mess in my head.