The XML sitemap is a document created to help search engine crawlers find our pages, URLs easily from a single place.

There are two types of sitemap
HTML Sitemap
XML Sitemap

Sitemaps are a protocol that allows the webmaster for a website to inform Google and other major search engines about URLS on a website that are available for crawling. Sitemaps allow search engines to find all of your webpages, that they might otherwise miss when indexing. The XML sitemap allows you to specify additional information about each URL such as:

When it was last updated
How often the site changes
How important the page is in relation to other pages on the site
Having this information within one document helps search engines understand your website and crawl it more intelligently. Sitemaps are an inclusion protocol, where Robots.txt files are exclusionary. For more information on robots.txt files please read our article.

You can provide multiple Sitemap files, but each Sitemap file that you provide must have no more than 50,000 URLs and must be no larger than 50MB (52,428,800 bytes). If you would like, you may compress your Sitemap files using gzip to reduce your bandwidth requirement; however the sitemap file once uncompressed must be no larger than 50MB. If you want to list more than 50,000 URLs, you must create multiple Sitemap files.

An XML sitemap is a bit of Extensible Markup Language (XML), a standard machine-readable format consumable by search engines.

XML sitemap is a document type that helps Google and other major search engines better understand your website while crawling it.

XML Sitemap is one of the best file to use to index website.