Two types of sitemaps are important for a website:
An HTML sitemap lists all content as a clearly structured, readable directory of clickable links of all pages and posts, similar to the main menu on a website. Plugins that can be used for this purpose allow sorting, including or excluding post and/or page content according to various criteria. For a fully customizable HMTL sitemap, you usually have to purchase the paid variant of the WordPress plugin in question.
The XML sitemap is the recommended format for displaying the hierarchical structure of a website, not only under WordPress. This makes the website more searchable for search engines, its so-called crawlability increases. XML is an abbreviation for “Extensible Markup Language” and consists of a text file, which is readable by humans and machines across all known web browsers.