When someone mentions a word or topic, you can say [word]と言うと to mean 'speaking of [word]' or 'when you say [word]'. It's a way to pick up on that word and add your own comment, often about what comes to mind or a nuance. It can also be used to ask for clarification, like 'when you say X, do you mean...?' In that case, the sentence often ends with a question or a statement of your interpretation.

You'll hear this in conversation when people are building on each other's words. It's not super formal, but it's not rude either. In writing, it's used to discuss the meaning or implications of a term. You can also use it in a rhetorical way, 'if you say X, then...' to introduce a logical consequence.