A Sitemap allows you to create a file hosted among the other files in your account that informs the search engines of all pages your website contains. In other words, it provides the search engine with detailed information about the content of your website.
Although Sitemaps are mainly used to improve the index process of your website by search engines, they can be used to provide accessibility to certain pages as well. For example, pages that are not accessible via your menus, can be indexed by the search engine if you want.
The Sitemap does not affect the normal way crawler bots index your page – it just extends the visibility and accessibility of your content.
Please note, that having a Sitemap on your website does not guarantee that a certain pages will be indexed and included by the search engines. Neither it affects your page rank.