What does Einstein’s famous equation, E=mc^2 , explain about the relationship between matter and energy?

Mass can sometimes be turned into energy.
Energy and mass can be converted into heat.
Mass cannot be turned into energy under any circumstance.
Energy and mass are the same thing

Question

What does Einstein’s famous equation, E=mc^2 , explain about the relationship between matter and energy? 
  
 Mass can sometimes be turned into energy. 
 Energy and mass can be converted into heat. 
 Mass cannot be turned into energy under any circumstance. 
 Energy and mass are the same thing

koikkara · Answer

It tells us that mass and energy are related, and, in those rare instances where mass is converted totally into energy, how much energy that will be. The elegance with which it ties together three disparate parts of nature—energy, the speed of light and mass—is profound. @noahbred