Einstein’s most famous equation tells us that there is an equivalence between mass and energy. As you probably know E= energy, m= mass and c = the speed of light
As the speed of light is already a huge number, the speed of light squares (c^2) is mind bogglingly large = 89875517900000000 m2/s2
So in other words there is a huge amount of energy locked up in even a very small amount of mass (the energy equivalent of 1kg is 25 billion kWh!!)
Of course, we cannot access all of this energy, so what use is it??
Well, apart from being the subject of every A-level physics exam paper for the last 50 years (!) it has a number of practical uses, including in nuclear power.
The nuclear reactors that we use to generate energy today are nuclear fission reactors – they take big atoms such as uranium and ‘split’ them, into much smaller atoms. When this happens there is a tiny mass change, and this mass is converted into energy according to E=mc2 – so the equation tells us how much energy we get out of our nuclear fission reactor (amongst other things!)