There could be a variety of reasons why someone is looking at you from the corner of their eye, but the most obvious answer is that they're attracted to you, but don't want you to notice that they're looking at you.
On the other hand, there could be a few more reasons for this, including the following:
- They're looking at you, but they're not attracted to you. Maybe they're just looking at your outfit, or perhaps you've got pen on your face or something.
- They're not actually looking at you, but instead, are looking at something in your vicinity. It might be a good idea to check that there's nothing interesting going on around you.
- They want to get your attention. Maybe they actually realise that you know they're looking at you, but want you to know that they're interested. It's likely that they've realised you're looking back at them.
Ultimately, it's most probable that they're interested in you and can't help but want to have a better look. Maybe you could try talking to this person, and try to see whether or not he appears interested or not.