A sitemap is a complete list of all the pages on a website, prepared in a tidy form so that search engines can discover every page and understand how the site is organised. It is not usually meant for human visitors — it works quietly in the background.
Think of it as the table of contents at the front of a book, or a directory board at the entrance of a large building: instead of wandering around hoping to stumble on every room, you get a clear list of everything and where it is. Search engines use a sitemap in the same way to make sure no page is missed.
A sitemap is a basic part of SEO: when search engines can easily find and read every page, the whole site has a better chance of appearing in search results.