One of Einstein's greatest gifts...
Was his ability to explain even complex issues in very clear and everyday English. He was a great teacher as well as a great thinker.
many other thinkers are almost inpenetrable but Albert was very readable.
It doesn't make it simpler to understand "what" in the real world the square of the speed of light represents though - it doesn't, really otther than when you also multiply by inertial mass and get a result with the dimensions of energy.
Nuclear power stations, radioactive decay and nuclear bombs attest to the correctness of the physics, even if we have difficulty understanding some of the concepts or breaking them down into sub-concepts.