The DOM is the browser's organized model of the parts that make up a webpage. It is like a family tree that shows which items belong inside other items.
The browser builds this structure from the page's HTML. A script can then find a button or paragraph in that model and update it when someone takes an action.
Most visitors do not need to think about the DOM. It is a behind-the-scenes idea that helps explain how a website can change without loading a completely new page.