What it includes:

Removing the ignition boots allows access to the spark plugs inserted into the cylinders. They are removed, and replaced. Generally along with the ignition boots and wires.

Why you need it:

Sparks produce the fire to get you're engine running and keep it running. With use, they wear down, and the spark they produce weakens, requiring they be replaced. This should occur every 100,000 miles, or as needed.