Spoiler alert for those who haven't finished the manga or the anime
After the Naruto and Sasuke fight (in Naruto volume 72: Uzumaki Naruto chapters 694-698), both of them lost one of their arms above the elbow. After a while, Naruto got a fake arm (I think Sasuke didn't) which was wrapped in bandages (it was shown in the rest of the mangas, The Last: Naruto the Movie, and Boruto: Naruto the Movie).
It was shown in the movies and in the manga that his arm can move and act like his arm used to before he lost it, so he can use it for eating, writing, carrying things, holding things, waving hand signs, releasing jutsus, etc.. But how does it work? I know it was created by Tsunade, and since she's an excellent medical ninja she could have found a way or something, but was it ever mentioned of how it worked or anything? I mean, I know the Naruto Universe evolved over time and stuff, so I guess they found a way to create body part that works like they used to.