Complex sentence: A catalytic converter (colloquially, "cat" or "catcon") is a vehicle emissions control device which converts toxic byproducts of combustion in the exhaust of an internal combustion engine to less toxic substances by way of catalysed chemical reactions.
Split into simpler sentences: A catalytic converter (colloquially, "cat" or "catcon") is an exhaust emission control device which converts toxic chemicals in the exhaust of an internal combustion engine into less toxic substances. Inside a catalytic converter, a catalyst stimulates a chemical reaction in which toxic byproducts of combustion are converted to less toxic substances by way of catalysed chemical reactions.