According to the Einstein's THEORY of Relativity, the mass of a body (SAY a particle) depends on the energy and on the momentum (say the velocity) with which the particle moves. 

So, we have a problem: is there a mass value that every observers can relate to? Yes: is the rest mass, that is the mass you could measure 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 coincide with the MINIMUM value measurable for every observers.

According to the Einstein's Theory of Relativity, the mass of a body (say a particle) depends on the energy and on the momentum (say the velocity) with which the particle moves. 

So, we have a problem: is there a mass value that every observers can relate to? Yes: is the rest mass, that is the mass you could measure 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 coincide with the minimum value measurable for every observers.