For balancing a chemical reaction, number of atoms on the right hand side should be equal to that on the left hand side
Let us take an example of the following reaction
H2O----->H2 + 02
in this equation, to it is needed to balance the number of hydrogen atoms and the number of oxygen atoms.
There are 2 atoms of hydrogen and 1 atom of oxygen on the left hand side and 2 atoms of hydrogen and 2 atoms of oxygen on the right hand side.
we need to add 2 on the left side of H2O on the left hand side and 2 on the left side of H2 on the right hand side.
these 2 gets multiplied with the number of atoms of the element beside which it lies.
Final bananced equation is-
2H2O----->2H2 + 02