Catalysts are substances which make chemical reactions go faster. This is very important if you are trying to make a financial profit out of a chemical reaction. Your chemical company needs a catalyst. The chemical reaction only needs very tiny amounts of the catalyst. Car exhaust pipes use catalytic converters to get rid of some of the nasty gases from the engines.
Some catalysts are very expensive e.g. the platinum in a catalytic converter.
The lead in leaded petrol will poison the catalyst in a catalytic converter. If you wanted to annoy your parents and your teachers you might consider bleaching your hair with hydrogen peroxide. Unfortunately your hair grows so the roots will come out in their original colour and you have to go back to the hairdresses to get it bleached again. This means the hairdressers will have to keep some hydrogen peroxide ready for when you come back. Hydrogen peroxide looks like water but it is not water.