Complex sentence: Electron-positron annihilation is the process that occurs when an electron which is matter and a positron which is antimatter meet and destroy each other, producing two gamma ray photons, of 511 keV each, in opposite directions.
Split into simpler sentences: Electron-positron annihilation is the process that occurs when an electron (which is matter) and a positron (which is antimatter) collide. They destroy each other and produce two gamma ray photons, of 511 keV each, in opposite directions.