Drupal interacts with Solr through HTTP. Drupal sends content into the search index via XML documents. It walks through a node and there's a library that converts the node's attributes to XML, and Solr indexes the XML. Solr accepts documents POSTed to /update, and a different POST can be made to delete. Searching is done via GETs.
If you set up Solr locally (you need Java installed) the server will run at the same default port that is specified in the module, so by installing the module and enabling it, Solr should work out of the box.