In simple terms, HTML5 is a Web page (or HTML document) in a plain text file that has been encoded using Hypertext Markup Language (HTML) to appear in a Web browser. Here is a meaning of HTML word-by-word:
Hypertext is a text that a user will click to jump from one document to another document. This is a reference to Web pages to link to one another.
Markup is the tags that apply layout and formatting conventions to plain text. In a simple way, the plaintext is “marked up” with the tags.
Language is a reference to the fact that HTML is considered as a programming language.