The lightbox effect has become increasingly popular on the internet.
When a visitor clicks on the image or the "larger image" link on a product page, it pulls up the larger image in a box with a link to close it - and fades the rest of your website behind the image in a black smokey screened effect.
Zen Cart version 1.3.8, 1.3.9
There are several core files that are over written as well as database changes.