Some people interpret this as, you can convert mass into energy. That is kind of wrong. The truth is mass and energy are equivalent.
For example, one of the most important discoveries of physics was that gravitational mass and inertial mass are the same mass.
Inertial mass is the mass that resists motion:
F= ma
Gravitational mass is the mass that causes gravitational attraction F = Gmm/r^2
And discovering that the same mass is used for both equations was a big deal. This is different from electromagnetic equations. Where we have charge for the force, and mass for the inertial resistance.
As it turns out, it isn't mass that resists motion, but rather it is all energy. That is, all energy gravitates and all energy resists acceleration. If you had an infinitely accurate bathroom scale, and put two bar magnets on it, if the magnets are aligned so that they are locked together (a lower energy state) the scale would read ever so slightly less than if the magnets were the opposite alignment. Getting such an accurate scale is problematic. But we can test this on larger objects.
And so, since all energy resists acceleration, as you try to increase an objects speed, the energy you add to it also begins to resist further acceleration. The object gains more and more energy, and hence more and more resistance to further acceleration. For this reason, no 'massive' object can move at the speed of light.
This once showed up in physics as the 'relativistic mass'. The relativistic mass is mass multiplied by the typical relativity factor. By using relativistic mass, all the previous newtonian physics equations become the more complex special relativity equations.
However, physicists nowadays don't like to think in terms of relativistic mass. They prefer to think of mass as a relativistic invariant (doesn't change with speed), and instead view it as the equations themselves are being corrected when we move from newtonian physics to relativistic physics.
