Stoichiometry defines the relationships between two or more substances that undergo a physical change or chemical change. Stoichiometry has a parallel number of atoms and molecules that are involved in a chemical reaction. Stoichiometry gives a brief explanation of the quantitative relationships between substances as they participate in chemical reactions.
This is illustrated in the image here, where the balanced equation is:
CH4 + 2 O2 → CO2 + 2 H2O.