The Backstage menu is what you will see when you start Object Manager or press the Backstage icon (also called the hamburger icon) in the upper left corner.

The Backstage is where to find your Account information and basic functionality for getting started, like joining a project or opening an existing one. After opening a project, and saving, you will get a thumbnail/shortcut to the project. The Backstage can be loaded at any time from within the user interface.