This module can show information about the server the site runs on — the operating system, the web server, the PHP version, the database version — alongside counts of the registered users and published articles on the site. Each of those blocks can be turned on or off independently in the module's options.
Server details are handy while a site is being built, but published to visitors they tell everyone exactly what software the site is running. On a live site it is worth turning the server information off, or restricting the module to an access level that only administrators can see.