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 comments switched off (or details/comments combined) and skip profiles if you have members switched off.

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 requires your CHMOD values to be configured right. The easiest way to configure them is to run a manual generation once first. Also 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 ready, give the url http://yourdirectory/mapindex.xml to each search engine (google's page is http://www.google.com/webmasters/sitemaps ). You should validate it first to be sure it's working.