Imagine you need to climb over a mountain to get from one town to another. A catalyst is like a tunnel through that mountain — it does not change where you start or where you end up, but it dramatically reduces the effort needed to get there. In chemical terms, every reaction has an activation energy — a minimum energy barrier that reactants must overcome. A catalyst lowers that barrier by providing an alternative reaction pathway, allowing the reaction to proceed faster and at lower temperatures. The catalyst itself emerges unchanged at the end, ready to do it all over again.
Your own body is a catalyst factory. Enzymes — biological catalysts made of protein — accelerate virtually every chemical reaction that keeps you alive. Without the enzyme catalase, it would take years for your body to break down hydrogen peroxide; with it, the reaction happens in milliseconds. The enzyme amylase in your saliva starts breaking down starch into sugars the moment food enters your mouth. These enzymes are astonishingly specific: each one fits its target molecule like a key fits a lock, and your body produces thousands of different types.
Industrial catalysts are equally transformative. The Haber process uses an iron catalyst to combine nitrogen and hydrogen into ammonia at practical temperatures — a reaction that produces fertilizer feeding roughly half the world's population. Every car with a gasoline engine has a catalytic converter, where platinum, palladium, and rhodium catalysts transform toxic exhaust gases (carbon monoxide, nitrogen oxides, unburned hydrocarbons) into less harmful carbon dioxide, nitrogen, and water. Oil refineries use zeolite catalysts to crack heavy petroleum into gasoline. An estimated 90% of all commercially produced chemicals involve a catalyst at some stage of their manufacture.