What is a sitemap?
A sitemap is a directory of urls for a websites, these directories are used for easier indexing of a websites by search engines. Because instead of crawling the entire site and keeping track of all the urls listed everywhere they can simply download the sitemap and know instantly all of the urls a siteowner would like to be listed in google.
This however does not mean that all links listed in a sitemap are immediately listed in google, other SEO tools are still important but sitemaps are a great tool in your SEO toolbox. All urls in the sitemap still need to be mentioned somewhere in the webpage and not blocked somehwere in your file
Over the years several additions to the sitemap protocol has been suggested such as video,image sitemaps the automatic generation of sitemaps for these kinds of content is debatable because according to google the video must be relevant to related to the content of the host page(Source) which is hard since a lot of pages nowadays use videos as addendums or tutorials, furthermore this could be solved a lot easier by using video markup.