A spark plug is a device for delivering electric current from

an ignition system to the combustion chamber of a spark-ignition engine to ignite

the compressed fuel/air mixture by an electric spark,

while containing combustion pressure within the engine. 

Your spark plugs are what supply the spark that ignites the air/fuel mixture,

creating the explosion which makes your engine produce power.

These small but simple plugs create an arc of electricity across two leads which are not touching,

but close enough together that electricity can jump the gap between them.