Sitemaps are used by Search Engines to figure out which pages of a site they should crawl.

There is a standard XML format [1] for sitemaps.

Google only allows 50k urls per XML file, so if you have more URLs, you need to split the urls into multiple sitemaps. Then you can create a sitemap index file and submit it to Google [2]. Or you can just submit a bunch of sitemaps to Google directly.