Complex sentence: The "Virtual-On" video games most prominent feature are large mechs called Virtuaroids (or "VR" for short), gigantic robots which the player controls in battles against other enemy VRs.
Split into simpler sentences: The "Virtual-On" video games' most prominent feature is large mechs called Virtuaroids (or "VR" for short). These are gigantic robots that the player controls in battles against other enemy VRs.