Uh, what? Goalposts. These are a really handy way to get lights in some different places than just putting them on light stands. But what are they?
Essentially, they are cross bars that make an upside down "U" in order to hang a light. "But why would I need to do that, Michael?" You might ask. And I might respond, "In order to have a light directly above where you need to be shooting, but don't want to arm it out using a C-Stand because the arm isn't long enough."
For short distances and lightweight lights, you can boom a light out over an area using the C-stand arm. For lightweight lights (or semi-heavy, depending on the grade of C-stand you have and LOTS AND LOTS OF SANDBAGS), you can also "super-boom"/"super-arm" the light out using the C-stands, as pictured below.
But if you need to put a couple lights out over something, and you just don't have the reach with the C-Stands, or they won't be stable enough, or you can't get the stands far away enough, then you can turn to goal posts. With a goal post, you can basically put up two high-highs (the giant C-stands with a full foot spread), and either rabbit ears or mafer clamps threaded through the knuckle of the c-stand. This will allow you to have a light over a wide doorway, or to hang a light in the middle of a room when you have a wide shot and need to hide the stand for the light and there is nothing on the ceiling to attach the light to. It's a really handy trick.
Just grab a couple of C-Stands, a couple of Mafer clamps, some speed rail, and you can make your own goal post. A word of caution though- use a safety chain on any lights you hang from the goal post as a back up fastener. The last thing you want is a light to fall off the speed rail from your secured Mafer clamp and bash an actor on the head, or break the light.