[racket] (no subject)

what does  the following part in the code do.

(module+ main
  (require racket/cmdline)
  ;; By default, use 9999 as the port.  This can be overridden
  ;; at the command line.
  (define current-port (make-parameter 9999))
  (void (command-line
         #:once-each [("-p" "--port") p "Port"
                      (current-port (string->number p))]))
  (start-server #:port (current-port)))
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