Enzymes are chemicals that catalyze reactions. They speed up a reaction by providing an alternate reaction pathway with lesser activation energy. Interestingly, enzymes themselves do not get consumed in the reaction. Here is an example pathway of an enzyme-catalyzed reaction:
S + E -> SE -> P + E
here, S is the substrate (or reactant), E is enzyme and P is the product.
The enzyme forms an intermediate compound with the substrate, which will result in the desired product, along with regeneration of the enzyme. Thus, the enzyme quantity stays about the same throughout the reaction. And hence, a small quantity of enzyme can carry out catalysis of a large quantity of the substrate. Also note that enzymes are highly selective in which reaction they catalyze.
Hope this helps.