In many reactions two reactants are mixed and one is used in excess. This makes sure that all of the other reactant is used up. The reactant that is completely used up is called the limiting reactant, because it limits the amount of products that can be made. Once it has run out, the reaction stops, even though some of the other reactant is left over.
You can find the limiting reactant using moles. Take Mg + 2HCl → MgCl2 + H2. Suppose 0.5 mol of magnesium is added to 0.5 mol of hydrochloric acid. The equation needs 2 mol of acid for every 1 mol of magnesium, so 0.5 mol of acid reacts with only 0.25 mol of magnesium. The hydrochloric acid runs out first, so it is the limiting reactant and the magnesium is in excess.
The limiting reactant decides the amount of product. Here 0.5 mol of acid makes 0.25 mol of hydrogen, which is a mass of 0.5 g (Mr of H2 = 2). If the limiting reactant is doubled, the amount of product is also doubled. If only the reactant in excess is increased, the amount of product is unchanged, because the limiting reactant still runs out at the same point. You can explain this in moles or in masses in grams.