O que significa a expressão "tit for tat"?

I was watching Friends as usual and something funny happened: Chandler walks into a room and unintentionally sees Rachel naked. After that accident, they can't get along anymore because Rachel won't get over the fact she's been so exposed. So Ross comes up with a very childish idea to try to solve the problem. He says that since he saw her boobies, he should also let her see his "peepee" [that's the term he used]. Rachel thinks for a second and then she agrees that's fair. So she says: C'mon, he's right. Tit for tat. Do you know what that means?

By the context, you must have figured its meaning yourself. But just in case, tit for tat is "an equivalent given in return, retaliation". It's much more common to see children using that expression. Just imagine a child hurts one of his friends and as a kind of repayment for that injury, the hurt kid will have the right to do the same thing. So he says: tit for tat! There are other expressions we could consider similar to that one: like for like; measure for measure; an eye for an eye; a tooth for a tooth; blow for blow. That's it for today guys. Take care!