The ignition coil is what generates the "spark" required to ignite gasoline. The explosion created is part of the process the engine uses to create power. If the ignition coil goes bad, then the vehicle won't start and an ignition coil replacement is needed.
Most older vehicles use only one ignition coil along with a distributor to distribute the spark to each cylinder in the engine. Most modern vehicles use one ignition coil, located on top of the spark plug, per engine cylinder
When performing an ignition coil replacement, disconnect the electrical connection to the coil and remove the hardware securing the ignition coil to the valve cover or manifold. On some vehicles, the intake manifold may have to be removed in order to gain access to the ignition coils.
Most manufacturer warranties will cover an ignition coil replacement up to a certain point. Then after the factory warranty expires, then there may be extended auto warranties available that will cover the cost.