Server Actions in React 19 and Next.js

Server Actions are async functions that execute on the server, not in the browser. They are marked with the 'use server' directive placed at the top of the file or at the top of the function body, signaling to the framework that this function must never run on the client side.

Because Server Actions run on the server, they have access to the full server context: database connections, environment secrets, session data, and filesystem resources. This is fundamentally different from client components, which can only access what the browser exposes.