First, they increase the rate of chemical reactions without themselves being consumed or permanently altered by the reaction. Second, they increase reaction rates without altering the chemical equilibrium between reactants and products.
Enzymes are biological catalysts. They speed up reactions although they are not changed in the reaction. Enzymes are proteins, and therefore are folded chains of amino acids with a specific shape. This shape is determined by the sequence of amino acids held together by bonds, for example Hydrogen bonds.
