The apache plugin queries the page generated by mod_status, parses it and submits the number of bytes transfered, the number of requests received, and the number of processes in the various states of the scoreboard.
The plugin uses libcurl to connect to the web server, optionally using a configured username/password to authenticate. It tries to keep a keep-alive connection open to minimize the overhead of connecting to the server. It's also possible to query hosts over SSL.
Details of the configuration can be found in the collectd.conf(5) manual page.