In many reactions one reactant is used in excess. There is more of it than the equation needs, so it is not all used up. This makes sure that all of the other reactant is used up.
The reactant that is completely used up is called the limiting reactant. It limits the amount of product that can be made, because once it has gone the reaction stops. Adding more of the reactant that is in excess makes no difference to the amount of product.
Example: in 2Mg + O2 → 2MgO, 0.2 mol of magnesium needs only 0.1 mol of oxygen. If 0.5 mol of oxygen is available, magnesium is the limiting reactant and oxygen is in excess. The amount of product is set by the magnesium: 0.2 mol of MgO, which is 8.0 g.
If the amount of the limiting reactant doubles, the amount of product doubles as well, because the reactants react in a fixed ratio. We can explain the effect of a limiting reactant using moles or masses in grams.