Leia Organa (Carrie Fisher) sacrificed her own life for that of her son, Kylo Ren (Adam Driver), in Star Wars: The Rise of Skywalker, despite briefly believing that he couldn't be brought back from the dark side. Kylo Ren's redemption was one of the main story arcs of Disney's Star Wars sequel trilogy, with each movie addressing whether or not it was possible to save him. The Force Awakens had Kylo kill his father, Han Solo (Harrison Ford), which to many suggested he couldn't be saved. Leia herself admitted as much in The Last Jedi, but Luke Skywalker (Mark Hamill)'s words that "no one's ever really gone" rang true for her in the end.
In J.J. Abrams' The Rise of Skywalker, Leia reaches out to Kylo Ren through the Force, calling to "Ben" while he's duelling Rey (Daisy Ridley) atop the ruins of the second Death Star. The act results in him pausing, and Rey stabbing Kylo with her lightsaber - though she ultimately uses Force heal to save him, in part inspired by her sensing Leia's passing - as Leia begins to die through the sheer effort of reaching out to her son. At this moment, Leia's body is already failing her, as a consequence of her using the Force in The Last Jedi. The novelization of The Rise of Skywalker reveals Leia's body had "paid a steep price" for that act of self-salvation, and Leia's life is slipping away from her before this final moment. She knows that the effort of using the Force to connect with Kylo will kill her, yet she does it anyway.
While The Rise of Skywalker presentation of Leia's death is a little muddled, the idea of Leia giving her own life to save that of her son's fits with not only that movie, but the whole sequel trilogy and indeed Leia's entire arc. Across all three of the Disney-era Skywalker saga movies, Leia has a fascinating connection with her son. In The Force Awakens, she strongly believes there's still good in him, asking Han to bring their son home. In The Last Jedi, viewers again get a glimpse of that bond, when Kylo senses his mother through the Force and opts not to take the shot that would potentially kill her (although it's too late to stop someone else from doing so). By the end of the movie, Leia's faith in the idea of Kylo's potential redemption has been restored again. And so when she gets the chance to bring Ben back to the light in The Rise of Skywalker, she understandably takes it.
As with the toll as the Force had taken on her body, Leia's death is better explained in The Rise of Skywalker novel, where she flashes back to the time when Ben was just a child, and she also interacts with the spirit of Luke, telling him: "I never gave up hope for him." That Leia would retain hope for her son is a resonant idea, since she is the character whose request for hope kick-started the Skywalker saga in the first place.
That it would come with a sacrifice, too, is a price she's understandably willing to pay, given - be it as Princess or General - Leia has always fought for what she believed in, despite the great risks. That she wouldn't ultimately do the same for her only child just doesn't fit with her character. For all Kylo Ren had often felt rejected by his parents, and Leia especially, this was the proof needed that he was loved. In many fantasy stories, a mother's love is an incredibly powerful thing, and here Leia knows it's the one and only shot to redeem Kylo Ren. Of course she uses it.
Leia saving Kylo Ren also ties into the larger themes of family within the franchise. While much of Star Wars is centered on the sins of the father, there's typically redemption in there too: Han gave his life to try to save Ben; it's touching that Leia does the same, and along with the memory of Han, together they were finally able to save their son. Likewise, Anakin Skywalker saved Luke from Palpatine, dying in the process. And while it's not quite the same, Leia's own mother, Padmé, died giving birth to her and Luke. Leia's death could've been handled better in The Rise of Skywalker, although Carrie Fisher's own passing undoubtedly made things trickier in that regard, but sacrificing her life to bring her son back to the light side is nonetheless a fitting end for her.