This is left in for older compatibility reasons, but is no longer used in Savvy. The internal search engine has been changed to no longer need reindexing.
It used to be used to reindex a site for the site's internal search engine. If you use Savvy CM's search tool, you will need to periodically reindex the site. Once choosing this option you will be presented with a choice of which templates to reindex. It is recommended to only reindex the templates that have had updates to them. Either in new pages or from updated content in existing pages.
This process can be resource intensive and take a few minutes to complete. It is best to only run it when necessary and only for the templates required.