A distributor cap is a component of the ignition system on many older engines.

For your engine to run, a mix of air and fuel are sprayed or injected into the cylinders. There, the air/fuel mixture is compressed by the pistons and ignited by the spark plugs. The rapidly expanding gasses that result from combustion force the pistons downward in their cylinders. The pistons then rotate the engine crankshaft in a way similar to how you might pedal a bike. But these operations do not happen all at the same time. Each cylinder must operate in succession. That means, each spark plug has to fire at a specific time so that the pistons work as a team to pedal the crankshaft.

Spark plugs, to do their job effectively, require a lot of voltage. Therefore, an ignition coil is used to increase the voltage coming from the battery or alternator. On many modern engines, the engine control module (ECM) sends a signal to a coil placed directly on each spark plug. But on other engines, especially older ones, there is but one coil, and it sends power to the spark plugs by way of a set of wires.

Between the coil and the spark plug wires lives the distributor that “distributes” electricity to each spark plug in its turn. On top of the distributor is a plastic rotor that spins in time with the crankshaft.

The distributor cap covers the rotor on top of the distributor assembly. The spark plug wires are attached to the cap. As the rotor spins inside the cap, electricity from the ignition coil is passed from the rotor to contacts placed all around the inside of the distributor cap, corresponding with each plug wire. Therefore, as the rotor spins, it energizes each spark plug through the distributor cap with power from the coil.