The lightbox effect has become increasingly popular on the internet.
What this modification does is when someone clicks on the image or the "larger image" link on a product page, it will pull up the larger image in a box with a link to close it and lightly screen down the site behind it making the image pop!
If you set it up in your configuration, the popup will also include any additional images that are displayed on your product page with a next> or <prev link to flip between the images.
This is for Zen Cart version 1.3.9, 1.5.0, 1.5.1, 1.5.3, 1.5.4, 1.5.5+, 1.5.6+
There are several core files that are over written as well as database changes.