Screwing with you, messing with you, joking around with and teasing you - all mean basically the same thing. It is usually done in a light hearted manner. Generally “making fun of someone” though is much more negative and can include being mean or bullying.

All of these things pretty much mean the same thing: "I'm just screwing with you" "I'm just messing with you" "I'm just playing around." They all imply that "I'm just making fun of you" and you shouldn't take it too seriously.

It means "I'm just making fun of you" or "I'm just playing around with you"