Transformational-generative grammar is a theory of grammar that aims to describe the implicit knowledge that speakers of a language have about the structure and formation of sentences. This theory, primarily developed by Noam Chomsky, highlights the distinction between surface structures, which are the actual sentences we hear or produce, and deep structures, which are the underlying meanings and grammatical relationships that generate those sentences. It emphasizes the rules and transformations that can convert deep structures into various surface structures, illustrating how humans can generate an infinite number of sentences using a finite set of grammatical rules.