A catalyst is a substance that makes a reaction go faster but is not used up. It is not a reactant and not a product, so it is chemically unchanged at the end. Its mass is the same before and after, and it can be reused. In an equation a catalyst is written above or below the arrow, not added to the reactants or products.
Hydrogen peroxide breaks down slowly into water and oxygen: 2H2O2 → 2H2O + O2. Adding a black powder such as manganese(IV) oxide, MnO2, makes oxygen bubble off much faster. In the practical you add the same mass of different black powders to the same volume and concentration of hydrogen peroxide at the same temperature, and measure the gas collected in a set time. The powder that gives the most gas is the best catalyst. To prove a powder is a catalyst, filter it off, dry it and weigh it: the mass is the same as at the start.
Car exhausts contain harmful gases such as carbon monoxide. A catalytic converter has a honeycomb coated with platinum, and the platinum makes these gases react much faster to form less harmful gases. The platinum is not a reactant and not a product, and it keeps working for years without being used up.