explain E=MC2?

All mass is made of energy, this is what was discovered during the early years of the 20th century. Prior to this, we thought mass was distinct from energy. We thought energy is simply the stuff that affects the way matter is moved. In the 19th century, we used to think that matter is indestructible, it can get rearranged but it always stays the same amount, which we called mass. We even had a law in physics called the "Law of conservation of mass". The mass never gets created or destroyed, just rearranged into new forms (solid, liquid, gas). But that law turned out to be inaccurate. With the sensitivity of mass scales we had back then, it seemed like mass never increased or decreased, but as the sensitivity got higher, and we discovered nuclear energy, it became evident that mass was being destroyed. Well, not destroyed, it was being converted into energy.

During a nuclear reaction, when you take two deuterium nuclei (version of hydrogen nuclei with one proton & one neutron) and smash them together, you will produce one helium nucleus. However, when you measure the mass of the helium nuclei vs. the original deuterium nuclei, you find that the helium weighs less. It's not a huge difference, only 0.6% difference, but it's consistent and measurable. When you calculate the amount of energy released vs. mass decrease, it corresponds exactly to this E=mc^2 formula. So destroying mass released energy.

So E is the energy released, m is the amount of mass that got destroyed. So what about c^2? Well, c is obviously the speed of light, which is just a constant of nature. So we see the term show up in a lot of different equations in physics, such as c, c^2, c^3, etc. It doesn't mean that the objects are travelling at the speed of light when they release their energy, it's just the multiplication factor. So mass is proportional to energy, and the speed of light squared is just the constant of proportionality. The speed of light shows up in a lot of equations in physics, not just with stuff dealing with light. It's better described as the speed limit of the universe, so everything is affected by the speed of light, not just light.

Similar to KE = 1/2 * m * v^2, only it is rest energy of a bit of mass.