What are SEF URLs?
SEF URLs are also known as Search Engine Friendly URLs, or Clean URLs. Most websites today are not built using pages but rather the pages the visitor sees are built on demand by using a Content Management System, or CMS, like Drupal. Since the pages are built on the fly by pulling content from a database, the human-friendly URLs are not quite as common, resulting in URLs that appear cryptic and are harder to remember. For instance, Drupal may create content such as http://example.com/node/1 or http://example.com/?q=home. Clean URLs will allow a more human-readable URL to be presented for both the visitors as well as Search Engines.
Where are the Clean URL settings in Drupal 8?
Drupal 8 differs from previous versions in that it has removed the clean_url configuration setting. Drupal 8 now uses URLs in a format similar to /node/1 instead of the old /?q=home style. Drupal 8 still allows you to create relevant path and URL names by setting up URL Aliases.
What is a URL Alias?
URL Aliases are still a form of Search Engine Friendly URLs, or clean URLs in that they can make the paths more relevant to both the visitor and search engines. For example, instead of having the About Us home page point to http://example.com/node/3 it can point to http://example.com/about.