Can the smart people of SW explain to me "e = mc squared"? Specifically,

First, what is energy? It's a specific quantity that comes in many forms. If you compress a coil spring, you're putting energy in it. If you accelerate a mass, you're giving it kinetic energy. Raise a weight against gravity, you're storing gravitational potential energy. Then there's electrical energy (watts*time) chemical energy thermal energy, etc.

Let's zero in on the mechanical energy: springs, kinetic, gravity. They are all tied together by applying a force thru a distance (the force & distance are in the same direction here; parallel vectors if you like).

So energy is force*distance. What is force? Newton says F=ma, mass*acceleration. What is acceleration? It's change in velocity per unit time. What is velocity? It's change in distance per unit time. SO. putting it all together:
velocity=dist/time,
acceleration=dist/(time²)
force=mass*dist/(time²)
energy=mass*(dist²)/(time²)
Can rewrite it as energy=mass*(velocity²) if we like.

What is a joule? 1 J = 1 kg m²/s²
Those units are kilograms, meters, and seconds, the MKS system.

Finally getting to Einstein, E=mC², the units are right because it's mass times velocity squared. The magnitude is AMAZING because C² is such a huge number. In MKS units, C=300,000,000 m/s ! Call it 0.3 of a billion! Square that and your numbers are astronomical!!