HTML stands for Hypertext Markup Language. It's a special language used for making websites. It tells your web browser what to show on the web page, like the words, pictures, videos, and other things. To do this, it uses 'markup' which are like special instructions written with words, numbers, and symbols. When you look at them, they might look a bit like a code, but it's actually a lot easier to understand than a computer code.
When your web browser reads the HTML instructions, it knows how to create the web page in your browser. For example, if the HTML instruction is "<h1>This is a Header</h1>", your web browser will know to create a large heading on the web page that reads, "This is a Header".
So when you look at a website, you're really looking at a web page made using HTML instructions. That's why HTML is such an important part of making websites and webpages.