It depends on the context, but usually when you find it on the web it's slang or very informal:
1) it could be something linked to facebook (you put a picture of me, or a link to my profile)
2) it could mean you suggest that the personne is going out with you (a deformation for "s'afficher avec quelqu'un"
3) it could mean you like someone (=tu me kiffes, tu m'aimes, as if you have a poster of me in your room like you're a fan) or in "tu m'affiches pas" could be I don't like you
and there are probably plenty of other meanings I don't know, as it's the kind of expression that people hear and don't understand and then misuse…