The reactants are the substances that start the chemical reaction, while the products are the substances that are produced due to the chemical reaction.
In the chemical reaction of the problem:
[tex]Zn(s) + S(s) \rightarrow ZnS(s)[/tex]
the substances on the left (Zn and S) are those that start the reaction, so they are the reactants, while the substance on the right (ZnS) is the one produced in the reaction, so it is the product.
