For centuries there was an argument as to whether 'natural laws' existed in other places besides earth. Nobody had ever been to Saturn, so how would you know 2+2=4 there? People really believed that the planets orbited in perfect circles, moved by the hands of invisible angels! In fact when Johannes Keppler figured out that the planets didn't move in perfectly circular orbits at constant speed, he spent years trying to figure that out, trying to square it with his religious education. He thought God must be trying to tell us something!
It was Isaac Newton who figured out that the planets behaved as 'falling bodies'--showing that it was nothing but gravity that made the planets orbit the sun. This was one of those discoveries that Changed Everything. The revelation that natural law was consistent throughout the universe came as an enormous shock! This one single discovery led to the beginning of what today we call 'science'. It even led to the creation of a new religion! Deism. Deists believed God doesn't intervene in the universe, that natural law is supreme throughout the universe.
E=MC^2 is similar. Up to about 100 years ago we thought there was only so much matter in the universe and only so much energy. You could create or destroy them, only change their form. Einstein went a step further to show that energy and matter were both made of the same thing, and , there was only so much of THAT, whatever it was. But you could transform energy into matter and back again.
Einstein didn't invent that, he discovered it. Just like 2+2=4, it was true in prehistoric times, and it will be true when the sun burns out. It's true on earth and in a galaxy a billion light years away. (That's about 7,000,000,000,000,000,000,000 miles, BTW). If Einstein hadn't discovered it, someone else would have, and it would be just the same ratio.
E=MC^2 is an expression of the ratio of matter to energy when switching. A tiny amount of matter makes a huge amount of energy. E is energy, M is mass, and C is the speed of light.