Reactants are the starting materials in a chemical reaction. They undergo a transformation to form products, which are the substances produced at the end of the reaction. In a simple equation, reactants are typically written on the left side, while products are on the right. In a chemical reaction, reactants can be elements or compounds. For example, in the reaction of hydrogen and oxygen to form water, both hydrogen and oxygen are the reactants. Understanding reactants is essential in fields like chemistry and biology, where they play a crucial role in processes such as metabolism and photosynthesis.