To understand Einstein's equation in practical terms, take a look at what happens when Hydrogen atoms fuse together under the intense gravity and pressure within the Sun.
4 Hydrogens make 1 Helium + ?
The important part to note in the last picture is the "plus energy" that is then depicted in the graphic above as gamma rays, which are a form of electromagnetic radiation, or in layman's terms, sunlight!
A small amount of mass was lost!
Now, you can see here that the mass of the resultant Helium atom is slightly less than that of the Hydrogen atoms that combined to produce it.
What does it all mean???
What this means is that the actual physical mass of the atoms undergoing nuclear fusion in the Sun is being converted into Energy! Most of this energy is in the form of Electromagnetic Radiation (light) which radiates out into space.
(Click the Earth for an overly-complicated diagram of our planet's thermal dynamics)
The sunlight that reaches the Earth is the energy responsible for heating the surface of our world, creating the climate and weather, growing all of the plant-life, and generally motivating nearly every living organism on the planet.
The moral of this little story is that Matter is Energy. You are actually made up of a vast amount of energy. In fact, if you weigh about 150 lbs. (68 Kg), you contain the equivalent amount of energy as 1,460,691,974 tons of TNT. That's one and a half BILLION TONS of TNT!!! You don't want to light that fuse...*