Fromage is the french word for cheese.
The fun part of it is saying it in context and deadpan sincerity. You say it in mixed company that includes at least one person who knows some french.
When they hear you say, “Oh well, c’est fromage” they instantly notice that your attempt to throw in a little french for conversation colour... failed in a way that is funny to hear, but could prove to be embarrassing if they point it out to you.
However, people don’t want to embarrass other people. But because “c’est fromage” enters the brain so quickly as a funny error, most people don’t have time to stop their little chuckle at your expense. Then they are kind of forced to explain why they are laughing at you.
So it’s an impishly fun way to watch people squirm a little with how to react to you as they try to figure out whether or not your mistake was intentional. It usually shows you how natural and honest someone feels while they are with you.
“C’est fromage? That’s cheese?”
“What did I say? Fromage? Oh jeez, I meant domage.”
“No you didn’t.”
“No, I didn’t.”