In a chemical formula, the part to the left of the arrow represents the reactants. The reactants are all the chemical elements that will react with each others to make the products (which will be on the right part of the arrow).
Example:
H2 + O2 --> H2O
H2 and O2 are the reactants (on the left part of the arrow) and H2O is the product (on the right side of the arrow)
Hope this helps! :)
