electron volts is a unit of energy. Using E = mc^2, one expresses masses in the equivalent energy quantity. It turns out that electron-volts is a convenient scale for the (energy equivalent) mass for elementary particles -- much better than trying to express the mass in kilograms.
Or energy in Joules, which are around 10^19 bigger. So you'd have the inverse of that everywhere as an inconvenient factor.
As it happens eV works nicely from meV up to TeV for particles in all the most useful contexts, from chemical bonds to particle experiments. You only start getting insanely huge factors when you stop being subatomic.
