That thing is a one way check valve. It lets oil into the pump to lube the elements but won't let oil mixed with gas go back into the oil galley.
Normally the engine will have enough oil pressure at idle to hold a constant pressure and keep oil pressure around the valve elements. If the pressure drops very low the valve will close preventing fuel from backing up into the engine oil.
There's a cone shaped valve similar to what's used in the CSV with a small spring behind it. If this gets stuck or seized no oil will flow into the pump and it will seize too or damage the injection elements. You can take the check valve apart and clean it. You should be able to push the valve open with very little force. I think it's less than 10 PSI.
