HTML
HTML is the language that says what is on a web page: headings, paragraphs, pictures and buttons.
In a simple example
A line like a heading tag around the words Our menu tells the browser to show Our menu as a big title.
Why it matters to you
Every website is built on HTML, so knowing the word helps you read what an AI builds for you.
Do not confuse with
HTML is the structure and the words. CSS is the look.