What does it mean when a guy calls you his girl?

I have been talking to this guy for one month online long distance everyday. Yesterday he called me and introduced me to one of his close guy friend on FaceTime to meet which I didn’t expect. Later on while he’s driving, he just says “She’s my girl” and it threw me off guard because I didn’t know where I stood with him. Is he serious about me and does it mean we’re seeing each other?
What does it mean when a guy calls you his girl?
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