And to answer the question that's sure to follow, a "glow plug" is a device that is used to ignite the fuel/air mixture inside the cylinder of the engine. A glow plug is generally scewed into the top of the motor casing, with a metal piece protruding from the middle of it. In order to start the combustion process, you need to use a battery-powered device called a glow igniter, which clips onto the glow plug to make an element on the bottom of the plug (which is pointed into the cylider out of your view) become red hot (hence the "glow" part). Once fuel is introduced into the cylinder and the prop is rotating and the piston is doing its up and down thing, the fuel is ignited by the glow plug. Once a constant ignition is established, the igniter can be removed. Thereafter, the platinum in the glowing element I mentioned before reacts chemically with the nitro in the fuel to sustain the ignition cycle, ideally until the fuel is exhausted or the pilot hits the kill switch (or loweres the throttle below a sustainable idle), cutting off the air and making continued ignition impossible. I know there are more detailed technical explanations, but this is the simplified version.