HTML stands for HyperText Markup Language. It's a tag-based, plaintext language similar to XML for creating the content aspect of websites. By contrast, the behavioral aspect of a website is made with JavaScript and the visual aspect is made with CSS. Together, all three are used together to build most websites we see today, although HTML is the only minimal requirement and websites can be made with only HTML and not XML.
HTML looks like this with tags made from angle-brackets around content:
<b>This content is bold</b><i>This content is italic</i>
In this case <b> is the start of the content that is bold and </b> is the end of the bold content. Similarly <i> and </i> are the start and end of the italic content.
"HyperText" means content that you can interact with (click on) to visit other pages with related content. Markup means to decorate the content with tags (to mark it up) to give it meaning.
HTML was created in 1989 when Tim Berners-Lee designed it to create a digital documentation system for scientists at CERN.