This is a genuine OEM replacement part, it sold individually.
The primary purpose of this part is to attach the chainsaw bar to the models shown below.
There are two of these Bar Mounting Nuts used on the saws.

This part is also used on Husqvarna and Craftsman Chainsaws.

This is an OEM replacement part and is sold individually.
Its propose is to deliver an electric current from an ignition system to the combustion chamber of a gas fueled engine.
It will ignite the air fuel mixture by the spark.
Spark Plugs will usually have to gapped to the manufactures specifications found in the owners manual.

Over time a Spark Plug will become fouled because the air/fuel ratio may be set wrong,worn piston rings, carbon build up and other factors.
It is better to replace a fouled spark plug, rather then trying to clean it.