A sitemap is a file, usually named sitemap.xml, that lists all the important pages on your website so search engines can find and index every one without missing any. It is a plain table of contents written for machines, and Google reads it to learn what your site contains.
For a small business, a sitemap is cheap insurance against being partly invisible. Without one, a search engine has to discover your pages by following links, and a page that nothing links to can go unfound. With one, every page you care about is offered up directly, which matters most for newer sites and larger ones where some pages sit deep.
A sitemap works together with your robots.txt file, which usually points to it, and it supports ordinary SEO by making sure the structured facts on every page can actually be found and read.
Every site I build generates a complete, up-to-date sitemap automatically, so a new page is offered to search engines the moment it ships. A search engine can only index a page it knows exists. See what goes into a full build.