it just means that the 4 buttons on the bottom are slightly dimmed (less bright than compared to stock) when they are on, btw you dont control them yourself although that would be cool
I answer a bit late (and a bit off-topic here ) just to say that with Gingerbread ROMs, you can use an app called YALT (free from the market) that allows, with a single press on a widget, to put the buttons light ON or OFF, pretty cool when watching a video to have buttons OFF.
Sadely, it has no effect if running ICS.