URLs, or Uniform Resource Locators, is a unique address for a web page. A URL contains a scheme (https) a domain name (example.com), sometimes a port (:80), a path (/path/to/my/file.html), sometimes query perameters (?key1=value1) and sometimes an Anchor (#SomwwhereInTheDocument).
In Backdrop, there will be a lot of interfaces that ask you for a path. This is most commonly the part of the URL that comes after the domain name. If your Backdrop site lives in a sub-folder on your domain, something like https://example.com/folder then the Path is what comes after the name of the folder in the URL.