Refers to URL rewriting, or the restructuring of a URL to improve readability and/or search engine optimization (SEO).
URL rewriting relates to the process by which the URL of a website can be restructured/masked from its relative path as dictated by its location in terms of directory and file structure, and replaced with a customized address string which when entered/browsed to, resolves to the same (original) content.
URL rewriting requires a rewrite engine, software located in a Web application framework running on a Web server that modifies a web URL's appearance.
Rewritten URLs (sometimes known as short, fancy URLs, search engine friendly - SEF URLs, or slugs) are used to provide shorter and more relevant-looking links to web pages. The technique adds a layer of abstraction between the files used to generate a web page and the URL that is presented to the outside world.