Fixed-width layout is an alternative way to edit your documents. As its name suggests, fixed-width allows you to display your docs in the form of a page. Not only does it have a sophisticated look, but it also allows you to more easily visualize how a doc will actually look on paper.
You can easily switch any document from fixed-width to normal view and vice versa by clicking View > Fixed-width in the menu bar.
The default settings for all new documents will be the setting you last switched to. For example, if Jack selected fixed-width, all subsequent new docs will be opened in fixed-width view.