Sitemap is a hierarchical representation of all the links that are present on your website. It comes with two extensions. One is with ".html" and one is ".xml". Both have their own purpose. The ".html" extension is for humans to easily be able to navigate through your website. The ".xml" representation is for the google web crawlers to be able to read the website links and index them.
To be seen on Google you need to add your WordPress website to the Google Search Console. What this means is that you submit your website to Google and google bots start crawling on your website. The bots go through every possible link on your website and index your entire website as it stores it on Google servers.