A sitemap is a file, usually at /sitemap.xml, that lists the pages of a website so search engines can find them without having to follow links to each one.
Each entry is a URL, optionally with the date it last changed. Search engines use it as a list of pages to consider, not a promise to index them.
You tell engines where it is by naming it in robots.txt and submitting it in Google Search Console and Bing Webmaster Tools.
Why it matters for a launch
On a new site with few links pointing in, the sitemap is the main way search engines learn your pages exist. Submitting it is one of the first things to do after launch.
Read more: Why a new website isn't on Google yet