Sitemap – SEO WIKI
A sitemap is an XML file that defines the structure of the site and the pages to be indexed by search robots.
Sitemap helps search bots correctly determine the location of a particular page of a site, the frequency and time of their update, the importance of a particular page, and so on.
Having a sitemap does not guarantee that all pages will be indexed. If some pages do not satisfy search algorithms, then they will be excluded from ranking.