Located under Tools > Widget Manager is the Space Admin Theme settings page for the widgets.
From this page, you can enable/disable Dashboard widgets and Metaboxes. Disabled widgets will not be available under Screen Options and are completely hidden for all user roles with access to the dashboard.
Reset All button will do exactly that, reset all settings related to the Widget Manager.
Upon the deactivation of the Space Admin Theme, all dashboard widgets will be re-enabled.