When someone places the mouse over an image, an overlay will be displayed with text. This grabs their attention and lets you briefly describe what the image says. You can also make it linkable so clicking the image or the overlay brings the user to another web page.
An example use of this would be a shopping site, where you can have images representing various product categories. When a user hovers his mouse over an image, he’ll see the category name and he can then click on the image.