Mass is a form of energy. The amount of mass-energy "stored" in an object is given by E = mc^2. So when the mass-energy is transformed into motion (kinetic) energy, the resulting system will have mc^2 amount of kinetic energy.
Note that the enery-mass relation is only true if the object is stationary or traveling at a relative low speed compare to light. In general, the kinetic energy for an object is