Bulkheads are used to attach plumbing to aquariums and sumps to allow water to flow through them.
Bulkheads come in different sizes and either with or without threads for plumbing to connect to. With threaded bulkheads you will need to use a male pipe thread to screw into the bulkhead to make a connection. Slip fittings will require solvent to bond the bulkhead to the plumbing permanently.