In a balanced chemical equation, the number of atoms of each element is equal on both sides of the equation.
An equation to be said that balance for a chemical reaction is that the number of atoms for each element in the reaction and the total charge is the same for both the reactants and the products.
For example, this equation for the reaction between iron oxide and carbon to form iron and carbon dioxide is unbalanced concerning mass:
Fe2O3 + C → Fe + CO2
This is a balanced equation for the charge because both sides of the equation have no ions (net neutral charge).
For balancing, the equation has the same number of atom on both the left and right sides of the arrow. This is achieved by changing the coefficients of the compounds (numbers placed in front of compound formulas).
Thus, the balanced equation is
2 Fe2O3 + 3 C → 4 Fe + 3 CO2