Hi 260,
It's the factory set bypass. When the valve is not actuated, that bypass defines your idle speed. Under the seal is a small allen-screw. Don't waste your time though, cause there's always a reason why you have an idle-problem, and it wont be the setting of that screw.
Clean out the valve with some carb-cleaner, then WD-40 treatment and ensure all other components work OK.
All the best,
Dokta