A lot of fans believe that The Mandalorian was the first story in that galaxy far, far away to canonically introduce Force-healing, but we’ve actually seen the ability in action in Star Wars: The Clone Wars before.
Throughout The Rise of Skywalker, Rey uses an unprecedented ability to transfer her life Force into others, thus healing them. While this essentially came out of nowhere, Disney was clever enough to bring the penultimate episode of Jon Favreau’s The Mandalorian forward, which featured a scene where Baby Yoda uses the Force to heal Greef Karga’s wounds. Seeing the adorable Child perform this deed made sure that people wouldn’t react strongly about Rey utilizing this aspect of the Force. But when you really think about it, it was Dave Filoni’s Star Wars: The Clone Wars that originally introduced us to Force-healing, all the way back in 2011.
In one of season 3’s chapters, referred to as the “Mortis Arc,” Anakin, Obi-Wan, and Ahsoka visit a strange planet that houses three individuals who serve as literal manifestations of the Force. The Daughter worshipped the light side, while the Son occupied the darkness. The Father kept the balance between these two until Anakin came along and shook things up. During the second episode, the Son kills Ahsoka and mortally wounds the Daughter. As a last act of good, the latter instructs her father to help Anakin transfer her life force to Snips, which brings the padawan back to life.
This even explains how Rey was able to understand this through ancient Jedi texts. Star Wars: The Clone Wars essentially confirms that this is a learnable skill, but since the three companions lose their memories of Mortis when they leave the planet, we don’t see it until two Force-prodigies perform the deed many years later.
Of course, it’s understandable how Rey was able to master this ability, but how could Mando’s sidekick do it in spite of, well, being a child? To get the answer to that question, I guess we’ll have to wait and see when the next season of Favreau’s show comes along and explains the enigma of Baby Yoda.