Burning fuel can produce several toxic gases. A catalytic converter is a device that takes those toxic gases and converts them into less toxic gases.
For example, it converts Carbon monoxide into Carbon Dioxide, and Nitric Acid into Nitrogen and Oxygen (and a few other reactions).
In order to do this, it needs some precious metals which act as catalysts for the reactions. These metals don’t get consumed, but their presence helps to trigger these reactions. A catalytic converter will contain Rhodium, Platinum, and Palladium.