What is the significance of einstein's famous equation e = mc2?
energy is not related to mass.
mass can be destroyed.
energy can be destroyed.
mass and energy can convert to one another.

Energy and Mass are interchangeable. That means the energy and mass are convert to one another.

Einstein submitted a special theory that develops a nuclear power, where  the energy become a mass and mass become an energy.

E = mc²      OR   E/c² = m

Where

E stands for energy, (Joules)

m stands for Mass (Kg) and

c stands for speed of light (m/s)

where speed of light is constant, therefore E and m are mutually convertible

Also we find that the units on both sides of the equation are balanced

resonant frequency

grav potential energy