Sitemap is a well constructed XML file that specify list of contents at your website.
Google and other search engines CAN crawl you site without sitemap. (Reference: Do I need a sitemap?) However, it is a nice way to organize your contents and also a way to let search engine know about your new or possibly not well linked contents.
And great article is at https://moz.com/blog/xml-sitemaps-guidelines-on-their-use