Server Components
Server Components = React components that stay on the server
React Server Components (RSC) are React components that run only on the server and whose code is never sent to the browser. They have been a stable part of React since version 19.
A Server Component can read directly from a database, a file or a secret, then sends the browser a serialized description of what to display. Interactive parts (a button, a form) remain client components, explicitly declared with the "use client" directive.
The main benefit: less JavaScript to download and run on the user's side, so faster pages. Next.js, with its App Router, is the most widespread implementation. The model changes how a React application is designed: teams have to decide, component by component, what lives on the server and what lives in the browser.
Why it matters when hiring
This is a topic for an experienced React front-end or full-stack developer. The good signal: the candidate can explain where they draw the server/client boundary and why, and has already hit the pitfalls (caching, sensitive data leaking to the client, incompatible libraries). Having "done Next.js" is not enough: many projects still use the older Pages Router model.