This addon allows you to display the urls used by the software in what is described as a more user friendly format. Basically it changes the urls from something like this for a category link:

http://example.com/index.php?a=5&b=232

to a url that is more descriptive of the contents like:

http://example.com/category/232/For-Sale.html

A listing details pages links from something like:

http://example.com/index.php?a=2&b=556

to a url that is more descriptive of the contents of that listing by incorporating that listings title into the url like:

http://example.com/category/233/Learning/listings/16/title-of-this-listing.htm

For more description of this feature look to the SEO Addon wiki page and for instructions on installing this addon look here within the support wiki.