We are talking about the concept of limiting reagent. If you have a reaction involving two or more reactants, one of the reactants is usually present in excess. You can figure out which reagent is in excess by looking at the moles of each reactant and comparing them to the balanced chemical equation. Take the moles of each reagent and divide the number by the reagent's coefficient in the equation. This compares the number of moles of each reagent in their proper ratio for the chemical reaction. Whichever number is smaller is the limiting reagent. All of the other reagents are in excess and will still have some remaining amounts at the end of the reaction. But the limiting reagent will be consumed in its entirety. So the number of moles of product produced will be based on the moles of the limiting reagent according to the balanced chemical reaction.