An XML Sitemap is a list of the pages on your website written using the XML protocol. By submitting an XML sitemap to a search engine the search engine spiders will come to know about all the pages on that website, including any pages that would go un-detected during its normal crawling activity.

XML sitemap acts as a route map for the search spiders to let them find what you want indexed on your website.

Every webpage that doesn't have a link from your website (Isolated/Orphaned pages) can also be indexed by search spiders, if they are written down in the XML sitemaps.

An up-to-date sitemap.xml file will ensure every web page on your website is listed on search engines with the latest content.

There are a number of free online tools that can automatically generate a sitemap.xml file for you.