The reactant in a chemical reaction needs to cross the energy barrier to get converted into the products. This energy barrier can be crossed by supplying sufficient amount of activation energy to the reaction.
Enzymes act as the biological catalyst. They reduce the activation energy barrier and speeds up the chemical reaction. Due to the reduced activation energy, the rate of product formation gets increased. At the end of reaction, enzyme is recovered in an unaltered form.
Enzyme consists of an active site, which the substrate molecule binds. Each enzyme is highly specific towards a particular type of substrate. Substrate attaches to the active site and brings about conformational changes in the enzyme active site. This binding produces an Enzyme-Substrate complex.
Thus, at the end of reaction substrate is convereted into the product and enzyme is obtained in an unaltered form to be available, for the next reaction.