It sounds like you're talking about the kind of rail system that's used for mounting video equipment. There's some detailed information here and a video here but in a nutshell a rail system is often the foundation upon which all kinds of video equipment is mounted. That includes the camera, of course, but also sound equipment, follow focus adjuster, matte box, additional displays and interfaces, lights, batteries, shoulder mount, tripod mount, perhaps even a teleprompter. The rods come in standard sizes (15mm or 19mm) and are arranged in standard configurations with respect to the center of the lens, so that equipment from various manufacturers works together.