Achemical equation is balanced by changing or adding
a.
chemical symbols.
c.
numbers in front of molecules.
b.
subscripts after atoms.
d.
reactants in the product.

A chemical equation is balanced by changing or adding C. numbers in front of molecules, which are called coefficients. 
For example:
O2 + 2 H2 ---> 2 H2O (number 2 is added to both sides in order to balance the equation)

It’s the energy source of stars