In short, it means he was probably looking at you because he found you attractive. When you 'caught' him looking, he may have been embarrassed. Perhaps he was a shy fellow. Or maybe he just didn't want to be caught 'eyeing you up.' It could really be taken in an innocent or creepy way. I'd personally lean towards innocent but keep wary of the possibility of creepy. I know I've looked away when a girl notices me looking at her but that's because I've always been the shy type. Alas. I hope this helps a bit and take care.