Bulkhead fittings are designed to allow liquids to flow through solid walls of a tank or other container. The fitting is made with three basic parts; the body, a lock nut, and a gasket.
The Body of the bulkhead fitting is threaded to fit the lock nut and has one end flanged out larger than the threaded part. The gasket goes between the body and the lock nut, to seal the fitting and prevent liquid from leaking.
To use a bulkhead fitting, one needs to drill a hole slightly larger than the outside diameter of the threads on the body part. The body with its gasket is placed through the wall of the tank or container, and the lock nut is tightened to seal the opening.
With a bulkhead fitting in place, you may now have liquids flowing in or out of the container in a convenient manner and without leaks.