Can someone tell me what this does exactly?

[Kt1906]

So I think this valve is called the air relief valve if I'm not mistaken. But does this black cap have to be on? The water pressure of the outtake is so much stronger with it not on but I do not want it to ruin the pump if it is not on. Can somebody please explain thank you

 

[1poolman1]

Looks like a valve handle. With the pump off, turn it and see what happens. Looks like you could put a finger or two in the fitting from the pool side and feel what it does.