Stoichiometry is important because it explains the relationships of reactants and products in chemical reactions. Without stoichiometry, it would be impossible to determine how much of a certain chemical is needed to yield a result or what molar amounts of each reactant and product are present in a completed reaction.
Stoichiometry means "element measurement" in Greek and is also known as Mass Relations. The principles of stoichiometry are based on the Law of Conversation of Mass, the Law of Multiple Proportions, and the Law of Constant Composition. In order to satisfy all three laws, every chemical reaction and corresponding equation needs to have a set molar ratio for its reactants and products that explains how it proceeds to completion every time. Once determined, the molar ratio for an equation does not change as long as the equation itself is untouched and no other chemicals are added to the reaction.
Because the calculations in stoichiometry are based on molar ratios, the molar masses of the products can be used to determine theoretical yields and limiting reagents. Stoichiometry can also help determine the concentrations of solutions that were used, provided that enough variables are accounted for. A proper stoichiometric molar ratio functions forwards and backwards so that calculating values for the reactants can be performed if the corresponding values for the products are known.