今天在看 Rack Gem 的源码时遇到一个问题，百思不得其解。

请问 mount 方法是干嘛用的？

rack / lib / rack / handler / webrick.rb

require 'webrick'
require 'stringio'
require 'rack/content_length'

module Rack
  module Handler
    class WEBrick < ::WEBrick::HTTPServlet::AbstractServlet
      def self.run(app, options={})
        environment  = ENV['RACK_ENV'] || 'development'
        default_host = environment == 'development' ? 'localhost' : '0.0.0.0'

        options[:BindAddress] = options.delete(:Host) || default_host
        options[:Port] ||= 8080
        options[:OutputBufferSize] = 5
        @server = ::WEBrick::HTTPServer.new(options)

        ## mount 方法的用途是什么
        @server.mount "/", Rack::Handler::WEBrick, app
        yield @server  if block_given?
        @server.start
      end

以下是文档的解释，但是我还是不太懂，求高人好形象解释一下，多谢。

mount

mount(dir, servlet, *options) Mounts servlet on dir passing options to the servlet at creation time

# File webrick/httpserver.rb, line 149
def mount(dir, servlet, *options)
  @logger.debug(sprintf("%s is mounted on %s.", servlet.inspect, dir))
  @mount_tab[dir] = [ servlet, options ]
end