Steve Marsden’s

In essence, a chemical equation is a shorthand expression of a sentence which describes the starting and ending materials. For example:

hydrogen combines with oxygen to form water

In symbol form this becomes:

H2 + O2 ® H2O
reactants      products

But there is a problem here. If we count the element symbols, taking subscripts into account, the numbers of each type of atoms are not the same on opposite sides of the arrows. We say this reaction is "not balanced" because it does not show conservation of mass. More importantly, it does not show the ratio in which hydrogen and oxygen actually react.