If you were at a cookout, making s"mores for many people, you would want to make as many s"mores as you could. You would continue making s"mores until you ran out of one (or more) of the ingredients (either the graham crackers, chocolate or marshmallows). The ingredient that runs out first limits the number of s"mores you can make. A chemical reaction is similar in that the reaction will continue until one of the reactants is gone. The first reactant to be completely used up is the limiting reactant.