I know what this means it's when something isn't a sex joke and when it does "That's what she said" is after someone says something that didn't sound like a sex joke and it becomes on
Me: Come inside!
Funny uncle: And that's what she said.
Me: *Laughs without knowing the joke.
Mom: How do you know that? You're only 7!
Me: That's what she said.
Mom: