The 2 inch Flapper Check Valve 2.5-8lbs. Spring. It is commonly used with, 2 inch rigid pipe and 2 inch flexible rigid pipe. Spaform Spas and others, this is a fairly generic part.
This part is plumbed in line to prevent water from travelling back in an unwanted direction. It is also used to restrict flow in a set direction and to encourage more water flow to travel a different route.