A bulkhead is a plumbing part used to mate a container with pvc pipe. By using a bulkhead you can drill a hole in your sump and/or tank and still have a water tight seal. In other words, bulkheads allow you to connect two comparments so that you can pass water through them, without letting water seep around the joint.
The bulkhead pictured here is an Oceanic brand bulkhead that will cost you about $15 at your LFS. I personally think that buying one is foolish when you can make one yourself for less than $2.00. As you can see in this picture, a bulkhead consists of three parts: a threaded male piece, a threaded ring (female), and a gasket. To make your own, all you need is a male bushing, a female bushing, and a gasket.