What is meant by the rest mass energy of an electron?

According to the Einstein's Theory of Relativity, the mass of a body (say a particle) occurs on the energy and on the momentum (velocity) with which the particle moves. So, we have a problem: is there a mass value that each observer can relate to? Yes: is the rest mass, that is the mass you could calculate in a frame of reference co-moving with the particle (in which the particle is still), that is the center-of-mass frame and that agree with the minimum value measurable for each observers.