The engine's distributor controls the ignition timing, which is when each individual spark plug fires.
The cap of the distributor usually contains the contact points within it, and can often be rotated to adjust timing. The cap also has posts where the spark plug wires can be plugged in.
Distributor Rotor Button
Usually just referred to as a rotor in the context of distributors, or a distributor rotor outside context, the distributor rotor button spins inside the distributor assembly and makes contact with each point as the connected spark plug needs to fire. It typically has one metal contact, but may have a weight on the other side to improve balance or a second contact in a waste spark system.