E = mc^2 is the equation which made Einstein famous. It has a greater significance. It solved many problems associated with the universe. It led to the invention of atomic bomb.
E - Energy
m - Mass
c - Speed of the light
It means that a particular amount of mass can be converted into energy and vice versa. Energy released during this conversion is equivalent to the mass of the object multiplied with square of the speed of light.
If we consider mass of hydrogen atoms in a glass of 1 litre water, it is sums up to 111 grams. If we want to convert this small mass into energy,
E = 0.111(Kg) * c^2 which is really huge amount of energy.