See http://www.sitemaps.org for an explanation of sitemaps, and http://www.google.com/webmasters/sitemaps for google-specific info.

Your SEO settings page allows you to generate sitemaps. You can either do so with a click when you desire or select to have them autogenerated. The generation process is intelligent enough to skip threads if you have posts switched off (or details/posts combined) and skip profiles if you have members switched off.

Please keep in mind that using sitemaps may restrict the number of URLs that will be indexed, for example the virtually-infinite calendar days won't be in the sitemap and any custom pages you make won't be in the sitemap. Which is not to say google won't spider them anyway, but it makes it less likely. Sitemaps are not for every situation, they're only for sites that want to focus spidering on specific important pages alone. But if you have a site with tens of thousands of topics and very few pages currently in google, then you have an ideal candidate for a sitemap.

The option to set priorities lets you decide which pages google should spider first (lower priorites may be skipped if google feels your site isn't worth full indexing). If you want to entirely remove one of the types of pages from your sitemap, set the priority to 0 or blank and it won't be included in the sitemap.

The option for frequency gives search engines an estimate of about how often the page should be expected to update.

The option for extra sitemaps is in case you have custom pages you want indexed. Note that this is only applicable to pages beneth the directory where your installation resides (if other sections of your site are a directory above, the search engines want you to create a seperate specification for that -- or you can let the rest of your site spider the normal way).

Note that autogeneration generates 50 items for every ten page views, so it updates more quickly on a busy site. It doesn't overwrite your old sitemap until it has all the data ready, though, so there's no need to worry about the sitemap being incomplete.

When done generating, your sitemaps index URL to give to search engines will be http://yourdirectory/sitemaps/mapindex.xml in WSN 8.1.0 and later, or http://yourdirectory/sitemaps/mapindex.xml in WSN versions earlier than 8.1.0. Google's submission page is http://www.google.com/webmasters/sitemaps if you want to be able to track errors, but WSN will automatically ping google with a notice of update when the sitemap is regenerated.