A sitemap is nothing less than an XML list of pages you want to be found on. So this list is actually a blueprint of your website and gives the search engines a better indication of exactly how your website is put together.
Through a sitemap, you let the search engines know which pages and files on your Web site you consider important. Furthermore, a sitemap provides important information such as when the page in question was last updated and how important it is. You can also indicate alternative language versions of pages.
A sitemap can usually be accessed via domain.co.uk/sitemap.xml.