Why do I need to know c’est-à-dire?

Because you may want to use an example to explain something, or to request a clarification of some sort.

What does it mean?

C’est-à-dire – roughly pronounced set a deer – literally means ‘that is to say’ but it’s used to introduce an explanation, or clarification, to a concept you’re trying to explain.

In English you might say ‘in other words’, or  ‘ie’ before your clarification.

You could also use the following phrases in much the same manner: à savoir, disons, entendez, je veux dire, seulement, simplement, or surtout.

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You can also use it as a question to ask someone to elaborate or explain something in more detail – you’ll often hear it in celebrity interviews when the journalist says ‘C’est à dire ?‘ to mean something like ‘how so?’ or ‘please go on’.

Use it like this

Je ne souhaite pas participer à la pièce de théâtre. C’est à dire que je ne me sens pas à l’aise pour parler en public – I don’t want to be in the play. That is, I don’t feel comfortable speaking in public.

Je suis passionnée, c’est-à-dire pas capable de m’arrêter – I’m passionate, which means I can’t stop!