One of the challenges of digital photography is getting correctly white-balanced digital pictures. Sunlight is a dramatically differently colored light than fluorescent lighting. Our eyes amazingly correct for this automatically - Our DSLRs are not quite as amazing.
The Wallace ExpoDisc attempts to resolve the white balance problem.
The Expodisc is a filter that mounts (or is held) in front of the lens. The lens is aimed at the light source for the picture you are attempting to capture and a shot is taken. The filter diffuses the light to yield an 18% gray digital picture.
Using the image as the basis for custom white balance in the Digital SLR corrects the white balance for that picture. This custom white balance shot can also be used during post processing.
I sold my Expodisc. I found that a cheap gray card did a better job for me. Some people swear by the Wallace ExpoDisc, others don't.