No content is added to the element.

Considering this HTML element:

<p class="element">
  Hello world
</p>

And this CSS:

.element:before {
  content: "Foo bar";
}

The text content will be prepended to the element's content.

Notice how the end result combines text from the HTML and text from the CSS.

You can insert images by using the url() function.

Considering this HTML element:

<p class="element" data-something="cssreference">
  Hello world
</p>

And this CSS:

.element:before {
  content: attr(data-something);
}

The element will grab the text content from the HTML attribute.