It means you’re covered in ants.
It also means he obviously likes you. But I’m going to give you the same advice I’ve given dudes about this same topic. If someone you’re talking to lightly touches your hand or knee, it’s a sign that person is comfortable with you. This is positive. Don’t think of it as a green light, however. I’ve known far too many guys who interpreted an offhanded stroke of their arm by a chick at a bar as an invitation to throw his tongue down her throat.
It’s not a green light. It’s a yellow light. Proceed with caution. This is better than a red light. If he’s touching you, it shows that he’s relaxed around you. He could be building up to asking you out. Or he could just be warming up to building up. I don’t know. I find far too many dudes these day are either in-your-face pick up car salesman with moves and lines they cribbed from a poorly written book or shy little gummi worms, sweet but limp.
Try touching him back the same way he’s touching you. Try it, then report back.