In Star Wars: Rise of Skywalker, Rey (Daisy Ridley) shocked everyone by unleashing a blast of Force Lightning, a powerful ability typically attributed to practitioners of the dark side of the Force like Emperor Palpatine. As a heroic character who draws her strength from the light side of the Force, it's worth wondering how Rey was able to use this ability.
During a critical moment in the ninth and final installment in the Skywalker saga, Rey and her allies are on Pasaana when the First Order arrive. Chewbacca is captured and seemingly taken aboard a transport. Rey tries to rescue him by moving the transport with the Force, but is interrupted by Kylo Ren (Adam Driver), who engages Rey in a battle of wills as they fight over the transport. The struggles continues for a while, until Rey loses control and lashes out with Force Lightning and destroys it. Rey falsely believes that she has murdered her friend, Chewbacca.
There are a couple of reasons that could explain Rey's ability to use Force Lightning, and both of them could easily be true. Force Lightning, being the signature killing technique of Palpatine (Ian McDiarmid), has always been associated with the dark side. Palpatine has even said that "the dark side of the Force is a pathway to many abilities some consider to be unnatural", but what Rey is able to do in Star Wars: Rise of Skywalker casts doubt on the idea that Force Lightning is one of these abilities.
It's true that on the big screen prior to The Rise of Skywalker, Force Lightning has only been used by Sith like Palpatine and Supreme Leader Snoke (Andy Serkis), but that doesn't serve as confirmation that the power is exclusive to the Sith. The notion that Force Lighting is a Sith power was made popular by Star Wars video games like Knights of the Old Republic. In a canon audiobook, Dooku: Jedi Lost, Count Dooku uses Force Lightning while still a member of the Jedi Order. Like Rey, Dooku wasn't taught this ability.
It's also possible that Rey's heritage has something to do with her use of Force Lightning. As previously noted, this was Palpatine's most famous power, so she may have inherited a natural talent for it. Rey using Force Lightning could have been foreshadowing for the reveal that Palpatine is Rey's grandfather. Regardless, this was just one of several Force powers Rey showed off in The Rise of Skywalker. In the movie, Rey uses a new version of Force Heal, and used both sense echo and Essence Transfer. These are all examples that are demonstrative of how powerful Rey Skywalker is in the Star Wars universe.