What are the child-pages?
In WordPress, pages (not posts) can have a hierarchical structure.
So:
-> A parent page can have one or more child pages.
-> A child page is a page that “depends” on another one, organizationally (and often in terms of URL structure as well).

Practical example:
-> Parent page: About Us → URL: yoursite.com/about-us
-> Child page: Our Team → URL: yoursite.com/about-us/our-team

🛠️ How to create a child page in practice
When you create or edit a page, in the right-hand panel (block editor / Gutenberg), you’ll find the “Page Attributes” section (or similar, depending on the language settings):

-> Go to Pages > Add New (or edit an existing one).
-> In the right sidebar, look for the “Page Attributes” section.
-> You’ll find a dropdown menu called Parent (or similar).
-> From there, select the page you want to use as the parent.
-> Save or publish.

Done: that page is now a “child page.”