Basically it's a group of voice actors(Resident Evil, The Last of Us, Fullmetal Alchemist, Overwatch, Naruto) that play dungeons and dragons on live stream almost every Thursday night on Twitch. The first season of Legend of Vox Machina is based mostly on the Briarwood Arc which I think starts at episode 25 of their live stream. It's pretty much a reinterpretation of this arc into animation form. They did a kickstarter like 4 or 5 years ago to get two seasons made. The cast of Critical Role actually voice their characters in the show as well.