What do I mean #"by balance mass"#? Well, if I start with 10 g of reactant AT MOST I can get 10 g of product, and the chemical equation must reflect this mass balance.
For the (balanced!) combustion equation:
#CH_4(g) + 2O_2(g) rarr CO_2(g) + 2H_2O(l)#
There are 12 g carbon reactant, and 12 g of carbon product.......and you fill in the masses of hydrogen and oxygen.
The given equation likewise balances charge. Product and reactants are electrically neutral. When we write redox equations we try to represent electron transfer:
#Fe rarr Fe^(2+) + 2e^(-)#
#2H_3O^+ + 2e^(-) rarr H_2(g) + 2H_2O(l)#
And electrons are eliminated in the balanced redox equation:
#Fe + 2H_3O^+ rarr Fe^(2+)+ H_2(g)uarr + 2H_2O(l)#
In each instance charge and mass are balanced, as is required. See here for another answer to this question, and also here for this topic where it concerns corrosion.
I have written here before that we practise this stoichiometry, this #"garbage in equals garbage out"# principle all the time. If you buy something for #£3-75#, and give the vendor a #£10-00# note, would you have cause for complaint if you received #£5-25# in change?