Short for "Giga electron volts" which means one billion electron volts. An electron volt is the amount of energy required to move a single electron through a 1 volt potential. The rest energy of an electron is about 0.0005 GeV. The rest energy of a proton is about 0.938 GeV. The beam energies of the world's largest particle accelerators reach about 1000 GeV. To probe the string scale directly we would need accelerators which could read energies of 10^18 GeV.