mod_rewrite is a module running on most apache servers that allows you to do all kinds of transformations and redirects on your urls. The cryptic codes it uses are regular expressions
This tool looks at a number of different URLs for your homepage, and reports back on the response code given by each. To be completely safe, a 200 response should only be returned by one page. Other pages should return a 404 or 301. Anything else could be a potential duplicate content problem.