Permalinks are the permanent URLs to your individual weblog posts, as well as categories and other lists of weblog postings. A permalink is what another weblogger will use to link to your article (or section), or how you might send a link to your story in an e-mail message. The URL to each post should be permanent and never change.
Yoast Definition: Permalinks are an important part of your site as both search engines and visitors use these URLs to index and visit your site. The type of permalink you pick influences the way these two parties see and value your site.
This is a perfect example of a permalink as if I were to post a new blog with that custom permalink.
Are you wondering how you can change your permalinks in WordPress?