A timing light is a specialized strobe light used to check when the spark plug fires relative to the piston position. It clamps onto the spark plug wire of the number-one cylinder. Every time the wire carries current to fire the plug, the light flashes. By pointing this flashing light at the engine's main crankshaft pulley (harmonic balancer), the pulley appears to stand still. This lets you align the marks on the pulley with the scale printed on the engine block.
Correct ignition timing ensures the spark occurs exactly when the fuel-air mixture needs to burn for maximum power. If the spark fires too early, the engine will ping or knock, which can break pistons. If it fires too late, the engine will run hot and lose power. On older cars with distributors, you adjust the timing by loosening the distributor hold-down clamp and slowly rotating the distributor housing until the marks line up.