The guy in the listing gave a reasonable description of what they do:
"Ignition modules are usually found either in the distributor, on the distributor housing or in the engine compartment. It is the electronic control for the ignition system that works by receiving a signal from a magnetic pickup or a sensor in the distributor. The ignition module then uses the signal to open and close the ground circuit to the ignition coil to fire the spark plugs.
System: Inductive / Pick Up"