What it includes:

Replacing the Air filter is very simple, procedure that involves only getting access to the Air filter, removing, and replacing it with a new one, and reassembling the container.

Why you need it:

Air filters remove debris and micro particles from the air intake to prevent anything getting into the engine that shouldn't be there. This replacement should happen every ~12000 miles in order to fulfill its job, and still provide enough air flow to run the engine off of.