The reason why Goku and Vegeta become so powerful in Dragon Ball Super is that they are able to acquire and develop a unique quality of energy called god ki. Usually a prerogative of the Gods of Destruction and their Angels, god ki can also be obtained by mortals in rare cases, and this is what set Goku and Vegeta on the path of mastering their most powerful transformations, Ultra Instinct and Ultra Ego.
Dragon Ball Super pushed the power levels of the Dragon Ball franchise to a whole new height. As if alien conquerors, perfect cyborgs, and evil demons were no longer enough, Goku and his friends immediately had to deal with a whole new class of opponents: gods. The overwhelming power of Beerus forces Goku to acquire god ki through the Super Saiyan God ritual. Later, when Beerus and his Angel, Whis, become allies, Vegeta trains with the latter and learns how to use God Ki himself. This allows the two Saiyans to fight against the strong opponents that they meet during the course of the series, including the returning Frieza, Jiren, Moro, and most recently Gas.
However, the transition from normal ki to god ki was so quick that fans have almost forgotten about it, when this was actually a huge development in the series, which sets Goku and Vegeta apart from their fellow warriors. God ki is an exceptionally pure quality of ki used only by divine beings, that is incredibly difficult to use and grants an immediate boost in power. While Vegeta trained with Whis to obtain god ki, he realized that the secret is to increase one's power while keeping the ki inside the body, preventing it from leaking outside through the classic Dragon Ball auras. This is confirmed later in the series when Goku completes Super Saiyan Blue by learning to power up without letting his ki leak out of his body in an aura. God ki also takes different colors according to the users: purple for Gods of Destruction, red or blue for Saiyans, pink for the fusion of Goku and a Zamasu, a Shinling.
God Ki Sets Goku And Vegeta Apart From The Other Dragon Ball Warriors
The most interesting aspect of god ki is that, with the exception of Angels, god ki is a power that can be acquired rather than granted at birth. This proves that mortals can ascend to the state of gods: Universe 11's Toppo is training to become a God of Destruction, and Whis once suggested that Goku could take up Beerus' role one day. This concept comes to fruition in Goku and Vegeta's strongest transformations, Ultra Instinct and Ultra Ego, which make them similar to Whis and Beerus respectively, and are completely based on God Ki. This also sets Goku and Vegeta apart from other warriors who are on a similar power level, such as Gohan and Piccolo after the recent Dragon Ball Super: Super Hero movie. However, neither Beast Gohan nor Orange Piccolo makes use of god ki, which means that they are still some steps behind Goku and Vegeta.
God ki is one of the most interesting innovations introduced by Dragon Ball Super. Mastering its power is the basis for Goku and Vegeta's current progression, which took a completely different route from the classic Super Saiyan forms, and it's also the reason why the two warriors are currently among the strongest in the Dragon Ball Super universe.