Guys, the head nod of acknowledgement means?

What does the head nod up mean? I know it means like, "hey" or whatever but is it like a friendly gesture or does it mean you like someone? What does it mean if you do it with a neutral face? Or a smile? My crush who did it to me had a neutral face and he did it when I looked over my shoulder at him. He seems to only do it to girls. Sometimes his friends actually? What exactly is it?
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- - Girls, do guys do this to you?
Guys, the head nod of acknowledgement means?
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