It is part of the main drain. When I took the dust cover off, I see the cover (attached pic). Below that is a screen to catch debris. Below that is a single line that is connected to the drain. The previous owners dropped a plug at the bottom but didn't screw it in. I took the plug out and closed everything up. When I have the handy valve on half skimmer and drain, tons of air get into the pump. I'm thinking the cover doesn't have a good seal and may be leaking air. Any thoughts?