This means if it’s ok for you, it’s ok for me.
“You stayed out all night and didn’t tell me where you were”
“Well, you did that to me last week, what’s good for the goose is good for the gander”
It’s usually used in situations where perhaps someone flirts in a bar, so their partner does the same thing, to say “I can do these things too”.
It’s kind of like a warning, in a way. it’s talking of equal rights, but it’s asking for fairness.