in an automotive ignition system is simply a transformer
to convert the standard 12 volt current into spikes of high voltage
current reaching anywhere from 20.000 to 40.000 volts.
Current is supplied to the coils primary winding, generally 200+ turns of copper wire that bumps the 12 volts to about 250. This circuit is connected to a sensor in the distributor, either a points or hall effect system. This sensor is constantly opening and closing the circuit. When you adjust the engine timing this is what you are changing, when the circuit is broken in relation to piston and valve position.
Whenever the circuit is opened it causes a breakdown of the primary winding’s electromagnetic field. Each time the field collapses it sends a surge of electricity to the secondary winding, a hair-fine wire a few thousand feet long and wound 25,000+ times. This winding boosts the current to the extremely high voltage needed to ignite the fuel/air mixture in the combustion chamber. The current is then sent to the distributor rotor that passes it on to the spark plugs.