Essentially, the right roller is your gauge/dash lights. Without your headlights on, you can turn it up until the soft stop, in will be full 100% bright.
However, if you turn on your wipers (like say in rain conditions) your driving lights automatically turn on, which will automatically make your dash lights 50% dim. This will make it hard to see during the day, then you can roll the switch past the soft stop until it clicks to full on. Your dash lights will be at 100% again even with the driving lights on. Don't worry, if you leave it on full click, it won't cause the dome light to stay on or drain anything. Not that leaving your dome (or visor) light on could, since it has a 10 minute timer after you lock your doors.
The left roller is your ambient light, if you don't have the full ambient light set, it will be nearly impossible to see this light during the day, it essentially points down to the cup holder and can only be seen in the dark.