Short answers, pulled from the story.
The electronvolt is a unit of energy gained by an electron accelerating across an electric potential difference of exactly one volt. It carries the symbol eV and represents a specific amount of kinetic energy equal to approximately 1.6 times ten to the minus 19 joules.
In 2019, the International System of Units revised its standards to fix the value of one electronvolt as an exact number in joules. That numerical value matches the charge of an electron measured in coulombs.
Scientists attach prefixes like milli-, kilo-, mega-, and tera- to the base unit to allow measurement across vast ranges from millielectronvolts to quettaelectronvolls. These prefixes enable calculations involving individual electrons and protons without cumbersome conversions between macroscopic electrical units and microscopic particle energies.
A proton has a mass of approximately 938 MeV per c squared while an electron carries a rest mass of about 0.511 MeV per c squared. The Higgs boson has a rest mass energy near 125 GeV as measured at two separate detectors with a certainty better than five sigma.
Physicists use this small unit because atoms and subatomic particles operate on scales too tiny for standard joules. A single photon of visible light might carry just a few electronvolts of energy making it practical for describing interactions within the vacuum of space or inside particle accelerators.