An expression used (typically through text) as a way to find out if the person you are talking to is okay with what you just said. If they are okay with it, then the conversation can continue to build. If not, then you can confirm it was only a joke.
John: You, me, and your girlfriend should have a three way.
John: ...just kidding?
Ben: That would be really weird.
John: *hesitation* Yeah I know.