If the width is set to Full, all sections of your theme will expand to the entire page's width.
You can still individually change the width of some sections through their individual settings, but the width of most of the teams will be affected by changing this option.
if the width is set to boxed, the width of the sections throughout the theme will not increase more than a certain number. This number can be defined on the next option, "Boxed width value."
Boxed width value
The width of the sections on your theme will not get any larger than the value you select on this option.
This option is effective only if the global Width is set to Boxed. Or on the individual sections, if their width are individually set to "Boxed"