1. The problem is to explain Einstein's theory of relativity focusing on the famous formula. 2. The key formula in Einstein's special theory of relativity is $$E=mc^2$$. 3. This formula states that energy ($E$) and mass ($m$) are interchangeable; they are different forms of the same thing. 4. Here, $c$ is the speed of light in vacuum, approximately $3 \times 10^8$ meters per second, and it is squared in the formula. 5. The formula means a small amount of mass can be converted into a huge amount of energy because $c^2$ is a very large number. 6. This principle explains phenomena like nuclear reactions where mass is converted into energy. 7. In simple terms, mass can be seen as 'condensed' energy, and energy has mass equivalence. 8. This insight revolutionized physics by linking mass and energy in a precise mathematical way. Final answer: $$E=mc^2$$ shows the equivalence of mass and energy, a cornerstone of modern physics.