In a over simplified manner, E=MC^2 is just describing how much energy is in a certain mass.
To put this in perspecitve:
The velocity of light is 3x10^8 meters/second.
So 1 kg of material has (1kg)(3x10^8)^2=9X10^16 Joules............That is 9 with 16 zeros behind it.
This is theory and we do not convert mass into energy at the same ideal situation that this describes but there is a lot of energy contained in mass.