While your website has a unique domain name, this is only a part of the URL, which stands for Unique Resource Locator. The URL is the complete internet address of each single page or object on your website. Although explaining URL’s would need some more words, just keep it this simple.
Example: The URL of this website’s front- or start-page is https://creative-dragon-works.com. The URL for my Web & Media page would be https://creative-dragon-works.com/web-media.
When creating pages in WordPress, you have the option to assign a parent-page to it. So, if I have my page „Web-Media“ and make another page with the name „Domains“ to which I make „Web-Media“ the parent, the URL will be https://creative-dragon-works.com/web-media/domains.
A parent-page can have many children and provides you a way how to sort your content into sections. In olden times this was even relevant for the indexing of your website by the search-engines like Google, Yahoo, Bing and Co.