The DOM is a virtual representation of every HTML element on the page, where each element is an object that is nested within the Document object, hence the term Document Object Model.
The W3C standardizes everything related to the DOM. Their specifications ensure that all browsers can work with the DOM API with any language that is capable of doing so, not just Javascript.
The browser will parse through an HTML document (AKA the page source), and render each element as a node within the DOM. Once the DOM is fully rendered, it becomes a dynamic environment that can be manipulated by scripting languages like Javascript.
The DOM is what allows programming languages to interect with HTML elements within the browser. The Document object comes with different properties and methods that make this functionality possible.