What does it mean when a guy stares at you?

What does it mean when a guy stares from a distance but doesn't smile (not on a rude way or anything)

Cuz I've been catching this guy staring at me when he's like 20 metres away from me and he doesn't smile but has a normal face.

(I mean it would be just weird if you looked at a girl and smiled creepily right??)

And if he's rude to you when he's around his friends but decent when you're alone?

And if he stares at you right in the eyes and doesn't seem the slightest bit nervous while talking to you?

Boys are so confusing