So unless you've spent the last few years never clicking on an image on a web page, you're quite familiar with Lightbox-like effects (and if not with the name then at least with the effect itself). Here's an example of Lightbox2 for those unfamiliar with it.
When I say "Lightbox-like," I am referring to any kind of "effect" that allows for embedded pop-ups of images and other forms of media (i.e. flash videos, etc). The benefit of Lightbox, aside from the aesthetics, is that it doesn't result in an actual pop-up (which are often blocked by a pop-up blocker on a modern browser), or require the user to navigate to another page and then hit the browser's "back" button to navigate back to the main content. The basic idea is that Lightbox makes a visitor's life easier, supposedly.