Stage is used to display content exclusive to the people on stage.
When you are in "Stage" view, you can edit the apperance of your stage displays. You can add multiple slides/layouts with the "New layout" button in the left panel. In the right panel you can click on any of the options to activate them, or change the style of any selected. And resize them in the middle.
Right click on a stage slide for the option to disable it, or move all connected devices to that one.
When you are ready to connect, see this.
If you have multiple stage layouts, you will have to choose one of them, this will be remembered for that device. When inside a stage view in a browser you can click on the screen to reveal some buttons to go "home", or to toggle fullscreen.
On the right side, if you select "Slide" you can change some options, like the background color. Here you can also set the output if you have multiple outputs and want the slide to show a specific output, instead of the first enabled output.