If one of my admire has a new girlfriend but he keeps admiring me, what does that means?

Answer 1

It means he wants to get to know you more, but not like a girlfriend, since he already has one.

Answer 2

It means that although he is with his girl for some reason, he still enjoys noticing other girls. Perhaps he'd rather be with her. Perhaps he'd rather be with you but didn't ask for some reason. Perhaps he likes you both equally but knows that he can only have one.

Be careful when you feel flattery from guys like this. Their attraction to you is probably real, but they usually care more about entertaining their own egos than thinking about someone else's feelings (either yours or his girlfriend's).