Answer is: includes the coefficients that are needed for balancing the equation.
It is important because in that way chemist can see lowest possible number of reactants in chemical reaction needed to form product of reaction. Coefficients with the lowest ratio indicate the relative amounts of substances in a reaction.
For example, chemical reaction: 2H₂ + O₂ → 2H₂O.
It can be seen that there are two molecules of hydrogen, one molecule of oxygen and molecule of water in reaction.
