The page side or sidebar is a layout element that is located along the side of the main area (ie content with header and footer).
In this area, you will find generally navigational component such as the page explorer (a list of sibling or descendants pages)
By default, the page side design is implemented by the template.
Note that not all templates support a page side.
By default, all templates that are not for a index page should support it. You should refer to the documentation of the template to confirm it.