Some chemical reactions go to completion, resulting in all of the reactants becoming products. These reactions are said to be irreversible.
For example, burning methane in oxygen is irreversible. All reactants become products, shown by an arrow pointing right:
Other reactions do not go to completion, in which case there is chemical equilibrium between reactants and products. These reactions are said to be reversible.
For example, the Haber process in which nitrogen and hydrogen are the reactants and ammonia is the product. Reactants and product exist in equilibrium, shown by reversible arrows:
The extent to which reactants become products is indicated by the reaction's equilibrium constant.
Since the reaction is reversible, it is possible to begin with ammonia and get an equilibrium with nitrogen and hydrogen. In this case, ammonia would be the reactant and nitrogen and hydrogen the products.