its a hard concept to grasp... but ill attempt to make a concrete example, imagine you have a cieling fan with 5 settings
1.off, no energy, nothing is happening
2.low, there is some energy, but you can still see through the fan
3.medium, there is a lot of energy, you actually have to concentrate to be able to see that it is not a solid circle as opposed to the orbiting blades
4.high, even more energy, there is no real way to telll through observation that it is not one circle
5.speed of light, according to e=mc^2, the massive amount of energy is basically the exact same as mass
once again, it is not a simple concept, i cannot even say that i fully understand it, as quantum physics is not fun