Gamma radiation consists of photons, like visible light or X-rays, but much more energetic. Visible light has an energy of the order of an electron volt (1 eV), a unit used by physicists. X-rays have energies from a thousand to millions of eV.
The High Energy Stereoscopic System (HESS) detects very high energy gamma rays, reaching a million eV (teraelectronvolts). There are few of these very high energy gamma rays: even for a relatively intense astrophysical source, the flux of gamma photons penetrating into the atmosphere is about one per month per square metre.