emacs 自动安装 package 包

下面是别人的配置，有些看不懂，请问 min-version 这个是什么啊，怎么我在 melpa 里面找不到这个包？

(require 'package)
(setq package-archives
  '(("gnu" . "https://elpa.gnu.org/packages/")
     ("melpa" . "https://melpa.org/packages/")
     ("org" . "http://orgmode.org/elpa/")))
 
(defun require-package (package &optional min-version no-refresh)
  "Install given PACKAGE, optionally requiring MIN-VERSION.
If NO-REFRESH is non-nil, the available package lists will not be
re-downloaded in order to locate PACKAGE."
  (or (package-installed-p package min-version)
      (let* ((known (cdr (assoc package package-archive-contents)))
             (versions (mapcar #'package-desc-version known)))
        (if (cl-find-if (lambda (v) (version-list-<= min-version v)) versions)
            (package-install package)
          (if no-refresh
              (error "No version of %s >= %S is available" package min-version)
            (package-refresh-contents)
            (require-package package min-version t))))))

(defun maybe-require-package (package &optional min-version no-refresh)
  "Try to install PACKAGE, and return non-nil if successful.
In the event of failure, return nil and print a warning message.
Optionally require MIN-VERSION.  If NO-REFRESH is non-nil, the
available package lists will not be re-downloaded in order to
locate PACKAGE."
  (condition-case err
      (require-package package min-version no-refresh)
    (error
     (message "Couldn't install optional package `%s': %S" package err)
     nil)))


(setq package-enable-at-startup nil)
(package-initialize)

(require-package 'magit) ;;;auto install magit