A glow ignitor is an electrical device which, on nitro-powered vehicles, clips to the glow plug and causes its heating element to glow yellow- to white-hot. This in turn ignites the fuel/air mixture when starting the engine. When the engine is running properly, the glow ignitor is removed. Most glow ignitors have a built-in rechargable nickel cadmium battery; inexpensive ones accept a standard C-cell alkaline battery.
There are also glow clips, generally used on small .049 Cox engines. These simply slide on to the glow plug and are connected via wires to a large 1.5-volt starter battery.