This part is a Replacement Watkins Water Check Valve that is 3/4"Barb x 3/4" Barb with Connect kit.. 

This kit is used to make the connection to existing  3/4" Clear to extend it on either side of the old chekc valve or to attach 1/2" Flex pipe which measures about 3/4" outside diameter..

You slide the 1/2" flex into the 3/4" clear and glue and clamp it and you would not use the barbed couplers in that case...  

This 3/4" water check valve is installed in the inlet side of the circulation pumps plumbing to prevent the back flow of water through the circulation pump once the jet pump is activated. All water check valves have arrows stamped on the outside to indicate the direction of flow which should be pointer towards the circulation pump. 

Troubling Shooting: The symptoms of a non-operative check valve are; * Water Leak - If water is leaking from around the check valve, it should be replaced. * Rattling - A rattling check valve indicates that water is not being allowed to flow properly past/ through the check valve. * Verify the check valve is installed with the flow in the correct direction. * Heater High-Limit Tripping - Poor water flow through the check valve could lead to heater high-limit tripping. Ensure the check valve is not block by debris and operating properly. * Surging jet pump (on spas equipped with a flow switch)