The expression c'est-à-dire or c'est-à-dire que literally means that is to say. It can be translated as "that is", "i.e." or "I mean (that)". The register being normal it can used in formal and informal contexts. Note that the expression is used to explain or expand upon something that was just said, or to ask for clarification. In informal writing, c'est-à-dire may be abbreviated to c-à-d, càd, or even cad.