Star Wars: The Rise of Skywalker introduced an apparently new Force Heal power - but it actually made its debut in The Clone Wars. Every Star Wars movie expands the lore, introducing new Jedi and Sith powers, and The Rise of Skywalker was no different. Its principal new Force ability was a variation on Force Heal that seemed entirely original.
Rey demonstrated Force Heal in the deserts of Pasaana, when she healed an injured desert serpent. According to Rey, she did this by transferring some of her life and Force energy to the snake in order to restore it; that's an approach that's never been seen before in Star Wars, and it fits well with the theme of balance. By the end of the film, Kylo Ren was demonstrating the same skill - presumably having learned it through his Force bond with Rey - and was able to use it to sacrifice his own life in order to bring Rey back from the dead.
But, surprisingly, this isn't actually the first time this version of Force Heal has been used in Star Wars. One of the most interesting stories in The Clone Wars sees Anakin Skywalker and Obi-Wan Kenobi summoned to the planet Mortis, where they encounter avatars of the dark and light sides of the Force, Son and Daughter. The plot is a subtle and clever one, serving as an analogy for the role of the Chosen One in bringing balance to the Force. An attack by Son leaves both Anakin's Padawan Ahsoka and Daughter herself mortally wounded; Daughter chooses to allow the last of her life energy to be transferred to Ahsoka in order to save her life.
But there is one notable difference between Daughter's sacrifice and the powers displayed by Rey and Kylo Ren. While Daughter is willing to give her life energy to save Ahsoka, she required a conduit in order to do so, presumably a being who represented the principle of balance. This may neatly explain why the power hasn't been seen before in the Skywalker Saga itself; recent Star Wars novels have hinted that balance is a distinct aspect of the Force, like light and dark, meaning it may well have unique Force powers associated with it. Rey has been learning from ancient Jedi texts that originate from the time of the Prime Jedi, who was dedicated to balance rather than exclusively to the light side. Rey fulfills the Skywalker role at the end of Star Wars: The Rise of Skywalker, restoring balance to the Force by killing Palpatine. What's more, at the end of Star Wars: The Rise of Skywalker, Rey's Jedi lightsaber is yellow; the color is muted from the Sith red and the Jedi blue, likewise suggesting that she's all about balance. All things considered, it's quite possible Force Heal isn't a light side power at all - it's a balance power.
While Star Wars: The Rise of Skywalker is the end of the Skywalker Saga, it's safe to assume Lucasfilm will eventually reveal the future of Rey's Jedi. That may well mean they unveil more new Force powers too, abilities exclusively associated with the aspect of the Force that is balance.