It is a common joke in America. It is usually used when someone is interested or likes someone but the other person does not seem to feel the same. “ This could be us” literally meaning this could be us, “but she playing” means the girl does not seem interested. ( This could also be the other way around with a boy)

What does but you keep playing mean?

Taken literally, the catchphrase would seem to indicate a great, even enviable, couple. (As in, “our relationship could also be ~this amazing~, if you stopped messing around.”) But in practice, it's typically used to caption weird, unattractive, nonsensical or otherwise unenviable images.

What does it mean when someone says you playing?

The expression “being played” generally means being taken advantage of by someone you care about.