What does Einstein's E = mc2 says :

Einstein's mass-energy equivalence (1905):

E = mc2, equation in German-born physicist Albert Einstein’s theory of special relativity that expresses the fact that mass and energy are the same physical entity and can be changed into each other. In the equation, the increased relativistic mass (m) of a body times the speed of light squared (c2) is equal to the kinetic energy (E) of that body.

What does it say?
Energy equals mass multiplied by the speed of light squared.
In other words ...
Mass is really just a super-condensed form of energy.
What did it teach us?
Because of the size of the constant in the equation (the speed of light squared, an unimaginably huge number) a colossal amount of energy can be released through converting a tiny amount of mass.
But was it practical?
Einstein's most famous equation hinted at the potential for the huge amounts of energy released in nuclear fission, when a large unstable nucleus breaks into two smaller ones. This is because the mass of the two smaller nuclei together is always less than the mass of the original big nucleus – and the missing mass is converted into energy.
The "Fat Man" atomic bomb dropped over Nagasaki in Japan on 9 August 1945 converted just one gram of mass to energy, but produced an explosion the equivalent around 20,000 tonnes of TNT.
Einstein himself had signed a letter to US president at the time Franklin Roosevelt recommending the atom bomb be developed – a decision he later regarded as the “one great mistake” of his life.