Why is #E=mc^2# important, and what does it tell us?
It is an important equation as it relates energy to mass. It changes the conservation of energy to the conservation of mass-energy.
It explains the energy emitted by radioactive decay and nuclear fission and nuclear fusion. It also explains the mass/energy change when a high energy photon, which has energy but no mass, splits into a particle and its anti-particle which both have mass.