H2 + O2 ---> H2O
The hydrogen are balanced, but the oxygens are not. We have to get both balanced. We put a two in front of the water and this balances the oxygen.
H2 + O2 ---> 2H2O
However, this causes the hydrogen to become unbalanced. To fix this, we place a two in front of the hydrogen on the left side.
2H2 + O2 ---> 2H2O
This balances the equation.