Coaxial cable carries a signal on a center conductor surrounded by insulation, a braided shield and an outer jacket. The shield makes it tolerant of electrical interference over long runs, which is why analog cameras and cable television use it. It carries video or radio frequency, not data and power the way network cable does.
Coax is the cable that built the first generation of camera systems, and plenty of it is still in service. Understanding its limits explains when an existing run can be reused and when it cannot.
Construction and interference
The shield surrounds the conductor and intercepts external electrical noise, and the impedance, normally 75 ohms for video, has to match the equipment at both ends. Poor connectors and kinked cable cause reflections that appear as ghosting or a weak signal rather than a clean failure.
Coax in modern camera work
Analog and HD-over-coax cameras still use it, and baluns can carry the signal over twisted pair for part of a run. Newer network installations use twisted pair or fiber, because those carry data and power. Reusing coax is a sound decision when the cable tests well and the camera matches the format.