The bar on the left side of the WordPress Dashboard is known as the Admin Sidebar. It is used as the Global Navigation Panel for Across WordPress Website. All the functionalities of Websites including plugins, themes, settings, and posts related are showcased here.
Admin sidebar shows complete functionalities but it checks the role of the users first. It is based on the user role. Different user roles will show different admin sidebars.
It is a collapsable menu for all screens. It can be customized from the User > Profile > Color Schemes.
You can hide tabs from the admin sidebar also. For that, some of these plugins might be helpful.
You can also edit your functions.php file to hide unwanted tabs from the admin dashboard.