By selecting a restrictive group, when a guest is detected as a search crawler it will automatically be assigned any "deny" permissions of this group in addition to the normal permissions of a guest. You can use this to provide lesser access to a search engine than you would a normal guest. You might for example wish to create a new group called "Spiders" and select that here. You could then deny permission for that group to view profiles to stop spiders indexing your members profiles.
Note: Spider detection is not perfect and can be simulated by users so this feature is not guaranteed to restrict content only to those search engines you have added.