The reactants are any substance, elements or compounds in chemical reactions.
So now, the question is what will happen to the reactants molecules when combined to the product molecules? The reactant molecules will be divided or split and will be merge with other molecule to form a new substance called a product molecule.
For example: The combination of gas substances like Methane and Oxygen (reactants) to carbon dioxide and water (products). Together they will form a new product molecule.