In general, this is a list of URLs for your website in a form of XML file (it also known as Google Sitemap because it was introduced first by Google). Besides that it allows you to include additional SEO-specific information about each URL such as the date it was last updated, how often it changes, and how important it is. XML format ensures that this information can be easily processed on different kinds of computers, applications and systems, so Search engines (Google, Yahoo, Bing, Ask, Baidu, AOL, Yandex etc.) won't have any problems with understanding your sitemap files.