You got it on the plugs and plug wires.
The rotor is the part in the top of the distributor under the distributor cap that distributes the spark to the cylinders in the firing order rotation.
It has a contact in the center where the coil delivers the spark and a contact point at the circumference where the spark jumps to the contact in the distributor cap.
The contacts where the spark jumps build up a deposit and the spark also erodes the edge of the contact and requires more voltage to get through so you replace the cap and rotor.
Sometimes I've been able to clean the deposits away and lightly file the contact edges sharp again to get buy for a while.
JD