E=mc2 basically refers to the equivalence of mass and energy. We tend to think of Mass and Energy as distinct objects . For example the computer keyboard has mass. We tend to think of energy differently. For example light carries energy. The flow of heat is the flow of energy. A stone at a height has potential energy
Relativity tells us that Mass and Energy are the same. The keyboard has an energy equivalent of the mass of the keyboard multiplied by the speed of light squared. Of course this is a huge number, not like the quantums of energy we are used to. It is this factor of c 2 that explains why this equivalence is not apparent. It is because of this that the change in mass of a block of wood when you heat it , is negligible.