When you're looking at a screen to quickly gather information, as opposed to editing the information, you generally want an uncluttered screen. On the other hand, even while looking at information, you sometimes need to make a change and need more editing "tools" available.
To make it easy to switch between "view" and "edit" modes, Voyager has a button called (Command) on the bottom toolbar. When pressed, it tells each item on the screen to switch between "view" and "edit" modes. "Edit" usually means showing more toolbars and buttons than "view". Some screen items ignore this (the Profile area, for example) and others change dramatically (the Chart area for example, see the two pictures below). The button is a toggle so pressing it twice switches back to the original state.
The initial state of Command Mode varies depending on what you're doing. For example, if you're using SmartPlan to edit a flight plan, it's reasonable to assume you want to edit so Command Mode begins on. But, if you're using a GlassView layout, the system assumes you want a clear and simple screen so begins with Command Mode off.