Ultra Instinct is a series of principles centered around allowing one's body to act independently and before any moves have been made. Goku is only one of a handful of fighters to have entered it, and even fewer have perfect it.Know more
Goku was never one to stand still for too long. After almost dying to the threat that Zamasu posed to various universes, he approached Whis for more training, as he wanted to become even better. A while back, Whis had explained to him and Vegeta the existence of another power level beyond anything they had achieved so far. Ultra Instinct. This form basically entailed allowing one's body to make choices independently of the conscious mind, thus greatly speeding the user. Furthermore, it was only the start of the greater power of the Gods.
Goku would first encounter Ultra Instinct in his fight against Jiren. After a prolonged conflict that debilitated him, he attempted to gather the energy of all Universe 7 for a massive spirit bomb that Jiren would ultimately return to Goku. He only survived by activating a form of Ultra instinct known as Autonomous Ultra Instinct that preserved him beyond death.
It would take Whis giving both Vegeta and Goku the insight to willingly activate Ultra Instinct Sign, the first step in the transformation. As explained by Whis. Ultra instinct is just the start of a well of power well beyond anything they had seen thus far. Perfecting that form would mean otherworldly levels of powers. The first time Goku entered this first state was against Jiren at the Tournament of Power. Through that Fight, Goku was also able to achieve a perfected Ultra Instinct form, though briefly as the form drained his stamina incredibly quickly.
A very potent version of Goku, much more focused on countering his opponents and dominating the neutral game. His auto combo is potent, combining good neutral potential and dominating pressure from his attacks. All of these can condition opponents to fall into a rhythm that Goku can then disrupt with fantastic defense. Nevertheless, there are drawbacks. For one, without assist he suffers a big loss of mixups, he needs resources to function at a good level, plus his lack of true strings into safe block strings enders makes him a very punishable target at times.