Stoichiometry is the study of relative quantities of the reactants and products in a chemical reaction. It operates on the law of conservation of mass, which states that the mass of the reactants on the left side of the equation equals the mass of the products on the right side of the equation. Once balanced, the ratio of the equation is formed using the whole numbers in front of each compound. The whole numbers are called formula units.
The coefficients also provide you with the mole ratio in the equation. Compounds for reactions are also given in moles because the number is smaller and easier to handle. The mole ratio tells you how much of the reactants yields the amount of products. For example, if the problem tells you the yield in moles of one of the products, you use the ratio of that product to the reactants to solve for the amount you started with in moles.
The units in your equation indicate if you are solving for the correct product or reactant. If you are starting with moles of A on the reactant side and you want to solve for the number of moles of C on the product side, balance your units. When multiplying your reactant A by the ratio, make sure unit A is on the bottom of the ratio so it cancels out the other unit A you are multiplying by and just leaves C, which is what you are solving for on the product side.
Balanced coefficients also indicate the amount of each reactant and product, relative to each other. When these compounds are dissolved into another neutral solvent, you know the amount of each because the ratio stays the same. If your ratios are given in moles, use the molar mass of each compound and convert the moles to grams to find the mass of other compounds in the equation. Grams are needed in experiments to measure the weight of reactants needed to yield a certain amount of product.