Fibers are stackful coroutines: Fiber::suspend() yields back to the caller, and $fiber->resume() picks up where it left off. They are the runtime primitive behind async PHP libraries.
You will rarely write fibers directly. Revolt, ReactPHP, and Amp wrap them with a familiar promise and async API.
Combined with FrankenPHP’s worker mode or Swoole, fibers let PHP handle concurrent HTTP and database I/O without forking processes.