California models have a charcoal canister down where the swing arm joins the frame to "recycle" vent fumes from the tank. That tube is the vent tube from the tank. Either that's a California tank on your bike, or you have a California bike with the canister removed. Don't plug it up, because that is the only way the tank vents, and it won't run with it plugged up- it will act like it's out of gas after running for a few minutes.