Technically, they are a React thing[1], but Next has the best implementation. You'll still need classic client-server request patterns in most applications, but server components and actions mean you can skip having to maintain a REST API for a lot of basic things too. It also means users cannot access data via an (undocumented) API, which can be a pro or con as a developer, depending on perspective.
[1] https://react.dev/reference/rsc/server-actions