Short answers, pulled from the story.
An electronvolt (eV) is the amount of energy gained or lost by a single electron when it moves through an electric potential difference of one volt in a vacuum. Under the 2019 revision of the SI, one electronvolt is fixed to an exact value in joules equal to the numerical value of the elementary charge of an electron in coulombs. It is not itself an SI unit, though the joule is.
The electronvolt was historically devised as a standard unit of measure because of its usefulness in electrostatic particle accelerator sciences. A charged particle gains energy proportional to the voltage it passes through, so the electronvolt captured that relationship in the simplest possible case.
A proton has a rest mass of 0.938 GeV/c2. In general, the masses of all hadrons are on the order of 1 GeV/c2, making the GeV/c2 a convenient unit of mass in particle physics.
In plasma physics, temperature is expressed in electronvolts by dividing the eV value by the Boltzmann constant to obtain the Kelvin equivalent. A typical magnetic confinement fusion plasma is 15 keV, which corresponds to 174 megakelvin.
A photon with a wavelength of 532 nanometers, corresponding to green light, has an energy of approximately 2.33 eV. The full visible spectrum spans a photon energy range of roughly 1.65 eV from red to violet.
The KM3NeT neutrino telescope has detected a neutrino carrying 120 EeV (exaelectronvolts), making it the highest-energy neutrino on record. For comparison, the Large Hadron Collider reached a proton center-of-mass collision energy of 13 TeV in May 2015.