Guys. When you call a girl 'beautiful', what do you mean exactly?

This is not a big deal, but I've been called beautiful from a good guy friend and I'm questioning what he means by that.
Do you know those lame questions people post as their stories on Snapchat? Like, "if you like me reply back with an 'x' and I'll post a picture of you explaining why I like you. But you have to answer some questions first." Well, I posted that question (I'm not very fond of posting these questions but I tried it anyways). He replied just like it was told and I sent him the set of questions. One of the questions was "describe me." And he answered back "caring, beautiful, funny." This was the first time I've ever been called beautiful, so I was somewhat shocked (and flattered) to see that word. We aren't 100% close, but we are friendly enough to give each other hugs and have a general conversation in the hallways. But I've seen him "stare" at me, it's not intense, it's more of a long look without saying anything. This mainly happens in the hallways, I always mistaken this for him wanting to say hi to me but never gets to type of thing. And when he sees me in the hallways, he always sees who I am with (normally my friends, for that matter). When I catch him, I see his eyes dart between me and the people I'm with, so I know haha.
Anyway, I know this is a genuine compliment from a good friend. So nothing romantic or anything.
Guys. When you call a girl 'beautiful', what do you mean exactly?
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