My understanding is that an electron volt is the energy an electron needs to move against an electrical gradient of 1V (imagine like rowing against the current). It is a unit of energy, so at first glance it doesn't really make much sense to use it a a mass unit. However, since e = mc2, you can calculate the mass that is equivalent to 1 eV (which, in kg, turns out to be about 10 to the power of -36).

The electron volt is related to the electric charge of the electron, not to its mass. Similarly, a magnet weights more than its strength turned into mass.