What does it mean when a girl smiles really big and waves?

There is this girl I know who always gives me a big smile and wave whenever she sees me. I could understand her doing this if we were close friends, but we really don't know each other that well. She will even say hi to me from a distance when I don't notice her. Is she showing interest, or is she just being really friendly?
What does it mean when a girl smiles really big and waves?
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