The balanced equation shows the ratio in which reactants combine. In practice, one reactant is often added in excess, which means there is more of it than the reaction can use, so some is left over at the end. The other reactant is used up completely. This is the limiting reactant.
When the limiting reactant has been used up, the reaction stops, even though the reactant in excess is still present. So the mass of product formed is controlled by the reactant which is not in excess, the limiting reactant. The leftover reactant has no effect on how much product is made.
Example: magnesium burns in oxygen, 2Mg + O2 → 2MgO. If there is plenty of oxygen, 24 g of magnesium forms 40 g of magnesium oxide. Adding even more oxygen leaves the mass of product the same. But doubling the magnesium to 48 g doubles the product to 80 g, because the magnesium is the limiting reactant. This only works up to a point: once there is more magnesium than the oxygen can react with, the oxygen becomes the limiting reactant.