What does that mean if the guy you like says hi to you and waves at you enthusiastically with a smile and then continues to smile with a big smile till you look away?

he likes u, or maybe have a crush on you

or he likes the girl next to you, or one of your friends, or the guy next to you.

or maybe it doesn't mean anything at all.

If you're interested in him, talk to him. Finding out someone isn't into you is a lot easier than the torture of not knowing. Suck it up. Hope for the best and prepare for the worst.