HTML describes what is on a page: this is a heading, this is a paragraph, this is a link, this is an image. Your browser reads that description and draws the page. Right-click any web page and choose "view source", and you are looking at HTML.
It works with two companions: CSS, which controls how things look (colours, fonts, layout), and JavaScript, which makes things interactive. Together they are the front end of the web.
Why you keep hearing it
Because it is the ground floor of everything online, and because AI has made it suddenly relevant to non-coders. AI tools generate HTML fluently, which means "can you make me a landing page" is now a sentence you say to a chat window. Email marketing has kept HTML in marketers' vocabulary for decades too ("HTML email" versus plain text).
What it means for you
You never need to write HTML by hand. But knowing what it is removes a layer of mystery: a web page is a readable text document, not compiled black-box code. That is exactly why AI is so good at producing and editing web pages, and why "just ask the AI to change it" genuinely works for websites.