Thor (Chris Hemsworth) began displaying his true powers of controlling lightning and thunder in 2017's Thor: Ragnarok after the death of Odin (Sir Anthony Hopkins), suggesting that Thor's powers were secretly being held back by Odin. Throughout the MCU to date, Thor has consistently ranked among the franchise's most powerful characters, famously claiming to be the "strongest Avenger" on several occasions. However, it is only after Odin's passing that his full force is unleashed, suggesting a connection.
Thor: Ragnarok featured the return of Thor's villainous sister, Hela (Cate Blanchett), who sought to rule Asgard and the Nine Realms through war and bloodshed. The climax of the film fulfilled the disastrous titular prophecy when Surtur (Clancy Brown) destroyed Asgard and Hela with it. At various points throughout the film, Thor exhibits powers he has never before used in the MCU. He only reaches his full potential, however, after Odin reveals to him that his hammer, Mjolnir, was only a tool meant to help Thor control his powers and that he does not need it to defeat Hela. Thor rightfully earns his godly status after this reveal, by single-handedly overpowering Hela and her undead army. This begs the question as to why did Thor not use these powers against Loki, Malekith, and Ultron earlier in the MCU.
The most popular answer to this question is that Odin was using his power as the Allfather to dampen Thor's abilities, the same way he forced Hela to stay trapped in her prison for centuries. This would explain why Thor only began to use those abilities after Odin's death, and why he never used them in previous battles, even when the Avengers were on the brink of defeat like in Avengers: Age of Ultron. There could be other explanations as to why Thor only began to use these powers in Ragnarok, such as receiving the Odinforce, but in either case, Odin's death unlocked Thor's strongest powers.
The explanation for why Odin might have blocked Thor's powers can be found back in the first Thor movie. Thor spent the majority of his adult life as a reckless prince who sought out battle against his father's wishes, which usually led to disputes that end in the loss of Asgardian lives. Odin could have chosen to dampen Thor's powers because he did not fully trust him, even after he proved his worthiness again on Earth. Additionally, Thor's actions in Thor: The Dark World, albeit heroic, went directly against Odin's commands and were explicitly labeled as treason by Heimdall (Idris Elba). Even though Thor stopped Malekith from plunging the universe into darkness, going against Odin's direct orders was probably the worst way to show Odin that Thor is a changed man.
While the theory that Odin was holding back Thor's powers is solid, there could be another explanation for Thor's sudden increase in strength in Ragnarok. As the rightful heir to the throne of Asgard and Odin's only biological son, Thor would also inherit the Odinforce, also referred to as the Allfather force, following Odin's death. The Odinforce could have amplified Thor's powers, allowing him to control thunder and lightning without resorting to a tool like Mjolnir to channel his powers through.
If Odin was truly holding Thor's powers back, it would further prove that Thor is indeed the strongest Avenger, since he was able to go toe to toe with the Hulk using a fraction of his power. Thor: Ragnarok made Thor one of the strongest MCU characters and a real asset to the Avengers, and his new powers were essential to defeating Thanos in Avengers: Endgame. Thor will continue to explore his newfound powers and possibly develop some more as he sets out on a new adventure in his fourth solo outing, Thor: Love and Thunder, slated for release in July 2022.
