What does it mean when a guy says"You're mine"?

So I've been seeing this guy, we get along really great. I stayed the night with him at his house(it wasn't the first time either) so were laying there on his bed, he grabbed me and pulled me close to him,while pulling me he said "your'e mine" and smiled. What did he mean by that?
Update: Oh and for all you wrong minded people we didn't do anything sexual!!
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