Div element importance

The div element is probably the most commonly used HTML element of all. **It’s useful for passing the CSS of its own class declarations down to all the elements that it contains.
I don’t understand the meaning of this…
Can anyone help?

It means that you can nest elements inside of a div and those elements will inherit the style rules defined for that div.

    .blue-text {
        color: blue;

<div class="blue-text">
    This text will be blue.
        This will also be blue because the color gets inherited from my div
    This will not be blue.