One of the ways that you can create a good looking button or clickable image is to set the background image of an element so that you can take advantage of features like :hover pseudo-classes. Sometimes you don't want the text to appear inside of that element, like if the text for your button is inside the image. The solution is using a huge text indent number (-9999, to take it off of the left side of the page). It's basically a trick. An em is just a measurement, equivalent to the width of the letter M.

You'll notice sometimes when clicking a link that you'll see the dotted outline text extend all the way to the left side of the page. That's because the text inside of that element is actually way far off 9999em's to the left of your display. Like a bad act getting pulled off-stage by the big long hook around the neck.