The original application of this tool was to get light into a firearm barrel in order to inspect the lands for damage and/or fouling but the tool has become eminently useful in a wide variety of shop applications.
Basically, it consists of a small, turned aluminum fitting that attaches a length of optical fiber to a small flashlight.
With the advent of high intensity LED flashlights these days, this little thing can put an amazing amount of light into tiny crevices. The older flashlight shown in the photo has since been replaced with a Streamlight Stylus Pro which I heartily recommend.
Streamlight 66118 Stylus Pro Black LED Pen Flashlight with Nylon Holster - Basic Handheld Flashlights - Amazon.com