Utilitronium is relatively homogeneous matter optimized for maximum utility (like computronium is optimized for maximum computing power). For a paperclip maximizer, utilitronium is paperclips. For more complex values, no homogeneous organization of matter will have optimal utility.
Utilitronium shockwave is a process of converting all matter in the universe into utilitronium as quickly as possible, which would look like a shockwave of utilitronium spreading outwards from the point of origin, presumably nearly at the speed of light.