E=mc²
The relation E=mc^{2} expresses the equivalence between mass and energy. If the mass m of a body is multiplied by the physical constant c (the speed of light in a vacuum) squared, then we obtain the energy. In certain circumstances, a mass m can be transformed into energy E.
This relation sometimes leads to false interpretations. It does not, for example, say that energy IS mass. It says that energy may in some circumstances be transformed into mass.
Note that this relation is the simplified version of another more general version, E^{2} = m^{2} c^{2} + p^{2} c^{4}, where p is the momentum. In particular, the relation E=mc^{2} applies to bodies at rest, and does not have much meaning if it is applied to a photon (which is not at rest in any reference frame and of which the mass is nil)...