An XML sitemap is a computer and human readable file that website owners can choose to place on their site to improve the indexing of their pages. Before any page on your site can rank for organic traffic, it must first be indexed by the search engine. Normally, a search engine spider will go to your homepage, and follow links on each page it visits to index as many pages as it can on your website, according to your link authority (a measure of the number & quality of links pointing to your site). The more link authority your site has, the more resources a search engine is willing to invest in indexing your pages.
Search engines support reading a XML site map to add to its list of pages to index. In addition, we can provide other important details to the search engine spider, like how often a page is updated, when it was last updated, and details about any images we want indexed as well. Providing a complete and properly formatted XML site map is considered a basic requirement by many SEOs, and will go a long way towards getting the vast majority of your pages indexed.