Activation energy barriers hold back spontaneous reactions. The amount of product produced in a unit of time is very small. So the rate of the reaction is very low. Catalysts lower the activation energy barrier. More product it produced, so the rate of the reaction is increased.
Catalysts increase the rate of a chemical reaction.
Catalysts are not permanently changed or modified during the chemical reaction so they can be used over and over again. The active site on an enzyme molecule is a very specific shape and will only bind reacting molecules that fit into that shape. All other molecules are rejected and cannot bind. An enzyme will, therefore, only catalyze one chemical reaction. Enzymes are very specific, cells make a different enzyme for each and every different chemical reaction.