The Page Container options allow you to set a fixed width for your total page content. This is a new feature in CE3. By default, we are designing at 1140-pixels, the new standard width for websites; previously 960-pixels was the standard width used by web-designers, but time marches on and wider resolution displays prevail.
There is absolutely no harm in going wide. If a visitor's display is narrower than your container width, then the page will adapt. This is called “responsive design”.
You can go completely full-width and fluid by setting Width to “auto”. Or if you want to design at a wider or narrower constraint, then set Width as “fixed” and adjust the numerical value to your liking.
Of course, you can set a background color. And you can use the sliders to create space, borders, shadows, etc. All of which is pretty self-explanatory.
Finally, bear in mind that individual pieces of the page may be constrained separately. Therefore, if you want your gallery grid to occupy the full-width of the browser window, but you want your masthead and navigation to adhere to a maximum width of 1140-pixels, you can do that. In such a case, set your Page Container's Width to “auto” so that your site will utilize the full width of the browser. Further on in the control set, you can use dedicated width constraints to manage the width of your masthead, navigation, block content, footer, etc.
To illustrate the impact of the Page Container width, here are two examples.
Page Container Width is “fixed” at 1140-pixels; page background-color (magenta) fills excess space:
Page Container Width is “auto”; the gallery fills all available space: