Looking at the chemical equation given above, one will notice that the equation is not balanced, because the number of molecule of reactants and product are not the same. To balance the equation, coefficient need to be added to the molecules as shown below:
4Al + 3O2 = 2Al2O3
Looking at the balanced equation above, you will notice that, at the reactant side and the product side, there are 4 atoms of aluminium and 6 atoms of oxygen. This is an indicator that the equation is balanced.