A chemical equation shows what happens in the chemical reaction. The basic chemical reaction can be written as follows:
Reactant → Product
Substance A + Substance B → Substance AB
In a balanced equation, the total number of atoms of each kind (e.g. A) in the reactants and products is the same.
The relationship between the different components of the reaction is referred to as the reaction stoichiometry.
Na + O2 → Na2O
The reaction above, between sodium and oxygen, is not balanced. We need to adjust the number of units of some of the substances until we get equal numbers of each type of atom on both sides of the arrows.
4Na + O2 → 2Na2O
The equation is now balanced. Be sure to double check the number of atoms on the left side of the arrow matches the one on the right.
You can read a detailed guide on how to balance a chemical equation here.