This option allows you to switch between a desktop or mobile design stage.
When working inside the builder, you'll see a couple of icons in the upper left-hand corner.
These icons toggle between desktop and mobile view. The desktop view will allow you to work with the same builder you already know. If you switch to the mobile view, the builder content area will reduce to 320px.
When you have the mobile view enabled, you can continue to design using all the same features and options. The only unavailable option is content area width. Keep in mind that the mobile design mode uses a grey backdrop that is not part of your design.
When working in mobile design mode, you can apply changes only to the mobile version of your design. This option is only available for a few design settings. You can identify these options by looking for the mobile indicator in the sidebar.
In the example below, you can see this icon next to the alignment option.
In the mobile design mode, content hidden from that view will appear translucent.
Similarly, you'll see the opposite behavior in desktop design mode.
Note that this behavior only occurs when visibility mode is enabled. If you'd prefer to see only the version of content intended for your design mode, you can turn off visibility.