A chemical equation is said to be balanced when there are the same number of the same type of every atom on both sides of the equation.
Balanced symbol equations show what happens to the different atoms in reactions. For example, copper and oxygen react together to make copper oxide.
Take a look at this word equation for the reaction:
Copper and oxygen are the reactants because they are on the left of the arrow. Copper oxide is the product because it is on the right of the arrow.
If we just replace the words shown above by the correct chemical formulae, we will get an unbalanced equation, as shown here:
Notice that there are unequal numbers of each type of atom on the left-hand side compared with the right-hand side. To make things equal, you need to adjust the number of units of some of the substances until you get equal numbers of each type of atom on both sides.
Here is the balanced symbol equation:
You can see that now there are two copper atoms and two oxygen atoms on each side. This matches what happens in the reaction.