Goku, the hero of the Dragon Ball series is notorious for constantly one-upping himself; just when you think he’s reached his peak, he gets even stronger. For a long time, the Super Saiyan technique has been Goku’s bread and butter, but believe it or not, he actually has another, even more powerful form.
Super Saiyan is a transformation that certain Saiyans (like Vegeta and Goku) and half-Saiyans (like Gohan and Trunks) can undergo to increase their power. The form is typically brought about by intense rage. For example, Goku first goes Super Saiyan in chapter 317 of the Dragon Ball manga during his fight with Frieza. Goku becomes enraged after Frieza kills his friend Krillin, leading to the transformation. In the years since, Goku and others have achieved higher levels of Super Saiyan, going all the way up to perfected Super Saiyan Blue, which was achieved by Vegeta in the manga Dragon Ball Super. This form is said to be the most powerful iteration of the Super Saiyan, but it still pales in comparison to the technique known as Ultra Instinct.
Ultra Instinct, also know as the "state of the gods," was first used by Goku in Dragon Ball Super during the Tournament of Power, a contest where champions from several universes are forced to fight one another. The losers’ universes are destroyed, so the stakes have never been higher for Goku, who is selected as one of the champions of his universe. Through training with Merus, Goku learns more about Ultra Instinct and is even able to temporarily achieve it during his fight with Moro. The technique separates the user’s consciousness from their body, allowing them to fight without being impeded by thoughts or emotions, which greatly enhances their power and speed. However, Merus and Whis later reveal to Goku that he was only able to achieve Ultra Instinct sign, an incomplete version of the technique, which is why Goku was only able to hold it so briefly. But Goku wouldn’t be Goku if he stopped halfway.
In the manga, Goku reflects on the teachings of all his past martial arts trainers, and by synthesizing everything he’s learned, he is able to achieve the technique’s full form, called Autonomous Ultra Instinct. Unlike Ultra Instinct Sign, Autonomous Ultra Instinct can be used for extensive periods of time. To properly use the technique, there are three steps that must be completed. First, the user must clear their mind of all distractions so as to operate solely on instinct. Second, they must break out of their “self-limitation shell” and unlock their inner potential. Step three is to combine the first two steps without losing clarity or control. It may sound easy when reduced to three steps, but Autonomous Ultra Instinct is difficult even for the gods to master, making Goku’s ability to learn it especially impressive.
The fact that Goku could not defeat Jiren with Super Saiyan Blue combined with the Kaioken technique, but was later able to defeat him using Ultra Instinct proves that Ultra Instinct is vastly more powerful. Don’t get too attached, though - it's only a matter of time before Goku tops that as well.