The electron-volt (eV) is a unit of energy used in particle physics: it is the energy acquired by an electron subjected to an electrical potential of 1V. Thus, we have 1eV=1.6 10-19J; it is a very small unit.. Multiples are the keV=103 eV, the MeV=106 eV, the GeV=109 eV etc.
From the relation in special relativity E=mc2 (c=299 792.458km/s is the speed of light in a vacuum), we can express mass in energy units. Thus, the mass of the electron of 9.1 10-31kg corresponds to an energy of 0.511MeV.