A spark plug ignites the air / fuel mixture in the combustion chamber by creating an electrical spark. The spark plug has positive and negative electrodes. The positive electrode receives an electrical charge generated at the ignition coil which jumps across an air gap to the negative electrode. The spark plug ignites the air fuel mixture to create an explosion that will push down on the piston, which turns a crank. This is much like what you do with your foot when you ride a bicycle. If the air gap on the spark plug becomes too wide (usually caused by wear of the positive electrode) the electrical charge may not be able to jump the gap. When this happens there is no spark. When there is no spark, there is no explosion, and no downward pressure on the piston. This is called a misfire. A misfire will decrease performance and fuel economy, and if left unaddressed, may cause damage to the converter. A good spark plug has a gap of the correct size, and a flat positive electrode with sharp (as opposed to rounded) edges. The spark plug is threaded into combustion chamber in the cylinder head. To replace it, it is simply unscrewed from the head. On some cars this is very easy, on others, access to the plug is obstructed.