That's the Smart Screen. It's letting you know that it's keeping the screen in because you're looking at the screen.

 

It's just a take on a well-known Samsung Feature. they call it 'Smart Stay'. actually quite nice if you have your phone set to time out and sleep at a short time interval. that way it won't turn off while you're doing something such as reading an email, or ebook, or looking at your photos, but will still turn off 1 minute or whatever later to save your battery.

Before one would have to manually reset the timeout setting awfully high and remember to turn their phone off manually if they didn't want their screen suddenly going off while doing something like reading an email. I must say the LG implementation works a lot better than the one Samsung uses, since the one Samsung uses is rendered inoperable in low-light or bright sun conditions.