In the "Expand mode", several elements, such as the Left Navigation Panel and the Controllers Row, are hidden from the main screen. This leaves more space to focus on the product data within the View itself.
To activate the Expand Mode, simply click on the "Expand mode" button on the top right corner of the screen (next to "Share" and "Export" buttons).
This mode is great for several purposes:
Presentation: this mode is great for presenting to your stakeholders or senior management; it hides the less important aspects of the screen, so that attention can be focused on the important stuff - data, items, and roadmaps. More items also fit in the screen, so less scrolling is needed to access the elements you most want to show.
Screenshots: as more data fits in the screen, one can take screenshots of the various views (e.g. Roadmap) and paste them into Powerpoint presentations, etc.
Editing: all editing capabilities are still active in this mode. This means that one can still work and edit their items. For example: when working in the Expand Mode in the Spec Editor, the Item Hierarchy Tree is now wider, so that more of the item text can be displayed on the tree and it is easier to edit.