Sitemap
A sitemap is a list of all the pages of your site, written for search engines.
In a simple example
It is a small file that says: here are my home page, my menu page and my contact page. A search engine reads it and visits each one.
Why it matters to you
It helps search engines find every page, including new ones.
Do not confuse with
A sitemap for machines is not the same as a visible Site map page for people.