The Add sub-page option allows you to create a new page which is not tied to a particular group. It might be used to show, for example, recent news, details of your U3A's committee structure, information about your locality, classified collections of links. Making sub-pages avoids the expedient of creating "fictitious" groups to hold miscellaneous material.
A sub-page will be associated with a top-level page or an individual group page, and will be reached by a sidebar link in the same way as an external web-site or uploaded document. The difference is that it will still be visibly part of the site, with the standard page header and layout.
When you select Add sub-page you will see the usual form in which to enter a page header and some text. You will also need to select the "parent" page: the one on which the link will appear. Saving the page will automatically generate a link from the "parent" page, where the "link text" is the same as the page header. Subsequently you may if you wish use the Change link option to add extra details about the link, and edit the link text. You cannot remove this sort of link yourself, but if you subsequently remove the page, the link to it will be removed automatically.
Sub-pages may be individually changed or removed via options on the Pages menu, in the same way as group pages. Their text can be formatted in the normal way. In principle they can contain any sort of content — links, events, pictures, contact addresses — but try to keep the connections logical: visitors to your site will not expect to find details of events via the Links page!