It's an impeller - the heart of your jet pump. Although a jet pump seems simple enough, there are some things you might want to know. Propellers work just like a paddle. They push against the water, just like the blade of a paddle does. Although an impeller sort of looks like a prop, it works very differently.
Lets start by describing the path of the water through the pump.
First the water enters the mouth of the pump on the keel of the boat where it is sucked in by the low pressure in front of the impeller. The shape of the intake path has an effect on the velocity and pressure of the water passing through it. The intake opening has about twice the cross-sectional area of the impeller. As the intake tract progresses toward the impeller, it gradually changes shape. By the time it gets to the front of the impeller, it has the same cross sectional area.