A spark plug, or sometimes simply called a plug, delivers the electric current from your Jeep's ignition system to the combustion cylinder to ignite the compressed fuel / air mixture. A spark plug consists of a metal threaded shell, electrically isolated from a central electrode by a porcelain insulator. It is connected to the ignition system with a spark plug wire.
The correct spark plug is critical to your engine operating at it's peak efficiency and delivering the highest power output. If you've made performance upgrades to your engine, a different spark plug may be necessary to accommodate the increased power output.