It means that he likes you. It is one strong sign that he might be into you and eventually feels something for you. The other way you'll recognize is if he looks at you repeatedly and then gives you the same look each single time. Another way is how he interacts with you, for instance, his eye contact.
He's shy and seems to have a big crush on you. He is admiring you but can't express into words so he shows it through his actions. If you like him too, you should give him a smile back and start a conversation. :)