The URL, Uniform Resource Locator, is the web address, for a page:

A page's URL is comprised of 3 elements: site URL + path of parent pages + page URL.

By default, Sites sets the URL to a new page's 'Title'.  However you may want to have a compact URL while using a longer title.

The URL of any page can be adjusted at: More actions -> Page settings

When visitors are navigating within your website, particular URLs are not particularly important.

However, URLs matter when publishing links to specific site pages from a newsletter, in documents or in email:
  • Legible URLs are desirable
  • It's important that the URL remain constant and working 
    • Note: rearranging pages in the site map changes the URL since the path has changed.