This option will hide admin menu sidebar with all admin menu items completely. You will not see the sidebar at all. Please use this option with caution, because your users will not be able to access the links in the admin menu after that. This can be useful in combination with Cusmin Admin Bar Editor, you can put all your custom links there instead, and this sidebar will not be needed anymore.
This will hide all default WordPress admin menu items. You would need to add your custom links in Admin Menu Editor after that.