Enter a URL
The redirect check 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.
If you are unsure as to why this might be a problem, try entering www.webdrive.co.nz into this tool. Notice how they have a number of "home" pages. The multiple copies of their homepage is competing with itself for links. If Webdrive used 301 redirects on all versions except www.webdrive.co.nz, their homepage would have more link power and would probably rank better. Webdrive are a great web host however.