A sitemap is an overview of all the different pages on your website. Sitemaps most often start with your homepage, where it shows what other pages it leads to. The sitemap then does the same for the pages shooting off of your homepage, and continues doing so until all your pages are covered. If you want to visualize a sitemap, then imagine a giant family tree, with your start page at the top, and then branching downwards with the pages it leads to.
Sitemaps might not seem very readable to humans, but they are a good representation of your website for search engines, which is why it's often used when Google crawls a page.