What does it mean when he touches you on the shoulder?

So this guy who is an acquaintance (we're fb friends but don't really hang out and haven't spoken to each other much) recently came up to me while I was sitting in class and grabbed my shoulder (gently) and said 'it's good to see you'. What the? What's that supposed to mean?!?! I'm in college btw.
What does it mean when he touches you on the shoulder?
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