A Sitemap is a list of all the pages (URLs) on a website and serves to give both search engines and users an overview of the content available, as well as the fundamental structure of the site. This will help them (better) understand how the website is structured.
There are different kinds of Sitemaps, all of which help search engines find and crawl new pages. This can, in turn, help with (faster) indexing of new content.
There are ways to automatically generate a Sitemap. This will happen either once a new piece of content is published or based on a specific timed routine.
You generally differentiate between two kinds of sitemaps – XML- and HTML-sitemaps – whereas the later is usually only called “Sitemap”.
An XML Sitemap is a structured format that a user doesn’t need to see, but it tells the search engine about the pages in a site, their relative importance to each other, and how often they are updated. HTML sitemaps are designed for the user to help them find content on the page, and don’t need to include each and every subpage. This helps visitors and search engine bots find pages on the site.– Wikipedia definition for Sitemap
XML- and HTML-sitemaps are important resources for Google, which helps them get to every piece of content on a website. You should therefore use both kinds of sitemaps whenever possible.