This determines whether to allow search engines to index your site. "Everywhere" will allow the search engines to search everywhere including login and signup pages, which means sites with Force Login turned on are still indexed. To avoid the risk of spam involved with the signup page being searchable, use "Everywhere except login and signup pages". "Nowhere" will tell search engines not to index any page. Note this is only a tag in the header of the site. It is up to the search engine to respect the tag.