This is a tool used by animators, artists, doctors, photographers and craftspeople to view and or trace transparent objects by way of a backlight. For those that do marquetry and create their own patterns it is very useful. It makes tracing of a design onto a transparent medium, such as vellum or tracing paper, much easier. There are dozens of commercial lightboxes available and one is displayed below.
These are usually available for the craftsperson in art supply stores. Smaller-size lightboxes also come in thicknesses as thin as 3/4" while some high-end products use sophisticated lighting and come with a large price tag especially with the larger models.
At the other end of the spectrum are those that are handmade. Two such examples can be found here and here. There is no shortage of these on the internet. A basic lightbox can be had by placing a sturdy box with the open end up. Lining the box with a white reflective material is a plus. Place a light source inside such as a worklight. On top of this place a picture frame with 1/4" translucent glass (plexiglass will work on smaller ones) in it. I know one fellow who made his own and seemed pleased with the results.
Personally I have not used one. I rely on the computer to digitally trace my patterns which I discussed in a previous post, but if I did not have this resource, I would certainly use a lightbox.