Thats one very long sentence.
An ozonizer is not really necessary but it can be useful, most people run tanks without them though.
It is basically a devices that exposes air to a powerful electric charge, thus generation the gas ozone (O3).
Ozone is a very powerful oxidizing agent and sterilizer, thus killing pretty much anything that contacts the gas.
Ozone can be dangerous to the tank if overdosed so it is highly advisable that it is run on a ORP controller.
Ozone needs to be administered in a reaction chamber, this could be a skimmer or a device made specifically for the purpose. And, the air coming out has to be filtered through carbon to breakdown the remaining ozone as it can be harmful to people.