Its your reserve gas/ gas off switch. Normally you turn it to 'off' when you're storing the motorcycle for a long time. Normal function will have it to 'on' . Then if you run out of fuel, you switch it to 'res' = reserve' and get yourself to a gas station quickly.
Don't forget to switch it back to 'on' after you fill up, otherwise you'll use up your reserve!
The reserve tank will hold about 2L or about half a gallon, but your manual will give you the specs.
Yep, that's your main fuel switch. Most motorcycles that are normally aspirated (have carburetors) will have that type of switch with 'Off', 'On', 'Reserve' positions. It's kinda a simplified fuel gauge. Always run in the 'On' position and when you run out of gas, switch over to 'Reserve' and find a gas station within 20-30 miles.
I'm in the habit of counting miles on my odometer. I always zero the mileage counter when I fill up and with a quick glance I can tell when it's about time to fill up before hitting reserve. With the Shadow, it's around 130 miles and with the Valkyrie it's about 120 miles. I rarely actually go to reserve...