So what is an MeV?

The standard unit of energy is the Joule, such that 1 Joule is equal to the work done by a force of 1 Newton in moving a distance of 1 metre (in direction of the force). However using this for particles seems a bit silly when everything occurs on such a small scale. Physicists have found that a more useful unit of energy is the MeV or 10⁶eV, i.e. Mega electron Volt or 10⁶ electron Volts - an electron volt? 1 eV is defined to be the energy required to move an electron through a potential difference of 1 Volt. As the charge on the electron is -1.6x10^-19C. it means that there are 6.25x10¹⁸eV in 1 Joule (or conversely 1 eV is 1.6x10^-19 Joules).

and a GeV?

Just as an M means 10⁶, so a G represents 10⁹ (Giga), which is useful as we head to higher energy physics