In technical terms, XML stands for Extensible Markup Language (XML). It is a standard machine-readable file format, consumable by search engines and other data-munching programs like feed readers.

In the simplest of terms, an Sitemap XML is a document that helps Google and other major search engines better understand your website while crawling it.

It basically lists the URLs (pages) of a site in a structured manner which allows you (the webmaster) to include additional information about each URL.