Learn French: Je travaille à la maison, c'est-à-dire dans mon salon. - I work at home, that is to say in my living room.

It translates literally to "it is to say", but in English we usually say "that is to say", "meaning", or "i.e.". While your current lesson focuses on sequential connectors like puis (then), c'est-à-dire is an explanatory connector. It connects two ideas by using the second to clarify the first. It is always written as a single set phrase with these exact hyphens!

Why do we use à for the house (à la maison) but dans for the living room (dans mon salon)?

À is used for general locations or concepts, much like "at" in English (you are "at home"). Dans means physically "inside" an enclosed space. Here, you are stating your general location first (à la maison), and then clarifying the specific room you are inside (dans mon salon).

Why is there a la in à la maison when English just says "at home"?

French nouns almost always require an article (like le, la, or un) in front of them. So while English drops the article in the set phrase "at home", French keeps it, literally saying "at the house" (à la maison).