Bulkheads are simply PVC unions which bridge the gap through a pond wall, a fish tank glass, a tank side, whatever.
Bulkheads come in three parts, sometimes four. There's the inside fitting which is threaded and goes through a hole you'd drill in the side of the tank, pond, vat, whatever. The rubber gasket fits around the base of the threaded male part and is INSIDE your tank (Turns out by water dynamics, it's easier to keep water "in" than it is to keep it from "coming out", so the gasket is inside the pond or tank.) You put the female ring on the outside threading tightly down (with or without a gasket) on the outside of the tank onto the male fitting coming from inside out.
Then you can interface any number of barbs, PVC fittings, tubing or what have you to the water tight bulkhead.
If you were ever to try and make a filter out of a big rubbermaid tool box or truck box, you'd need these for sure.
They're available in a variety of sizes. When you buy a 2" Bulkhead, it fits through a larger-than-two inch hole you've drilled in to the side of the tank. So, when we say two inch bulkhead, we're talking about the INTERNAL DIAMETER (The size of the pipe it's meant to take). Check out or enormously powerful and long-lived Secoh Air Blower from Japan.