What is mass-energy relationship? What is its significance?

Einstein unified mass and energy and said that one can be converted into another and the two entities are related by an equation.
The equation is given by,
                             E = mc²
where, m = mass that disappears,
E = energy that appears, 
c = velcoity of light in vacuum 
So, according to modern Physics, mass and energy are not conserved seperately, but are conserved in a single entity called 'mass-energy'.
Significance of this relation:
Many unsolved problems has been solved in Physics after the unification of this theory. According to this relation, as c is large; even a small mass difference can produce enormous amount of energy.