Coils convert the relatively low battery voltage to high voltage to fire the spark plug. A coil contains two sets of internal windings - a primary and secondary, both wrapped around an iron core. The primary winding is momentarily charged by the ignition module, but interestingly enough, high voltage in the secondary winding is NOT generated until current is suddenly halted in the primary. The collapse of the primary magnetic field then causes a high voltage surge in the secondary, and thus a spark to jump in the spark plug.
For general, installation and operating information, download this PDF ==> CoilPrimer.pdf