A Dragon Ball Z thing.
USSJ or Ultimate Super Saiya-jin is the level between SSJ and SSJ2. Gokou, Vegita, Brolly and Trunks all acheive it. The person's muscles increase in mass greatly and their hair becomes much thicker. The incresed muscle mass makes them extremely slow. (By Dragon Ball Zstandards anyway) It looks extremely cool but is not very useful against a fast opponent.
Totally at a loss as to what USSJ is or what SSJ is? In DBZeveryone has a powerlevel based on their ki or chi which is internal power based on your spirit. Ki is a real thing that is like a life force. In DBZ the main fighters have extremely high ki which allows them to fly, move fast, and fire ki blasts. Ki blasts like the Kame Hame Ha, are a channeling of one's ki from their internal power to the outside world. The ki blast can come from the hands, mouth, eyes, fingers, feet, or in Piccolo's case his antennae.
Back to USSJ, the race of aliens known as Saiya-jins in DBZ can achieve the level of Super Saiya-jin in which their hair turns gold, their eyes turn green, and they are surrounded by a gold ki flame. They become immensly faster and stronger. USSJ is a level after Super Saiya-jin (SSJ). The person becomes bigger and stronger but slower as stated before.