What do you mean by DOM and the importance of it?

The DOM, or Document Object Model, is an object representation of HTML.

HTML:

<body>
  <h1>Hello</h1>
  <button>Click</button>
</body>

is represented as a tree of objects.

Through the DOM, JavaScript can:

• find an element,
• change its content,
• add or remove an element,
• handle an event,
• change classes or styles.

Example:

const button = document.querySelector("button");

button.addEventListener("click", () => {
  button.textContent = "Clicked";
});

To be precise:
The DOM is not “the HTML itself”. It's a representation of the document that JavaScript can access.