Parent sections are regular sections, except for one key difference: they are used as a head/leader in order to group together similar sections. These grouped sections, under their parent, form a subsite. Each subsite can have its own name, main menu and look and feel.
Parent sections are used primarily to manage permissions for a group of sections. Importantly, we may want some users to see one subsite but not others.
The key customisation setting you may need to use is the "Parent section" dropdown.
You can choose from all available sections on the site, but it will be obvious which parent section you need to choose.
Example of a parent section: Global Cider