Are you talking about when there is a background image (like the top left logo) and the text within the block element is off the page by -9000px?

You shouldn't have images within your markup, not for that intended purpose. The idea is that you have text "my website", but instead of showing the text you want to display a graphical logo. So you simply create a box element the dimensions of your logo; set the image to the background and then indent your text -9000px off the visal page (idea being its a figure that most modern monitors will never see). Ideally the block is clickable to the "/" so make sure to turn the hover underline state off the text!

I know alot of people keep the text for accessibility and SEO purposes as well. I won't go off listing examples, but most standards compliant sites do this.

I wrote a tutorial on it here:

http://kirupa.com/web-tech/css_rollovers.htm

The real purpose behind this, from what I understand, is that our industry needed a way to stop using images for text. This is one of the solutions available to us. So instead of us having
<h1><a href="whatever.html?phpMyAdmin=4594f30712f4fabaff6997416810f3f2" title="something that makes sense and is good for SEO and accessibility"><img src="my-image.png" alt="some more relevant text" /></a></h1>
you get this:
<h1><a href="whatever.html?phpMyAdmin=4594f30712f4fabaff6997416810f3f2" title="something that makes sense and is good for SEO and accessibility">some text</a></h1>

with CSS you can set certain styling properties such as the h1's width, the a's display and text-indentation, etc...
point being is that its used so that we no longer conform to old 'standards' or techniques.

we can easily now give the presentation of images w/o embedding them on the page and giving the page more bloat.

here's an example:

h1{
width:300px;
overflow:hidden;
}
h1 a{
display:block;
text-indent:-400px;
background: transparent url(http://www.some-domain.com/img/some-image.png) no-repeat scroll 0 0;
}