You may sometimes hear about "new HTML5 elements", or find HTML5 described as a new version of HTML. HTML5 was the successor to previous HTML versions and introduced new elements and capabilities to the language on top of the previous version, HTML 4.01, as well as improving or removing some existing functionality. However, as a Living Standard HTML now has no version. The up-to-date specification can be found at html.spec.whatwg.org/.
Any modern site should use the HTML doctype — this will ensure that you are using the latest verion of HTML.
Until 2019, the W3C published a competing HTML5 standard with version numbers. In MDN docs and elsewhere on the web therefore, you may find references to other HTML versions, HTML5.1 for example. Since 28 May 2019, the WHATWG Living Standard was announced by the W3C as the sole version of HTML.