The Expodisc is a device that can be used to correct white balance for digital photography. When placed over the front of your camera lens, it allows you to create a "custom white balance" for your camera so that any image created on your camera in the same lighting condition set with the Expodisc will be perfectly white balanced.
Essentially, the Expodisc sets a reference for the camera as to what pure white light should be, based on the light that is traveling into the lens. This reference then is the new standard white as far as the camera is concerned. So as long as you take any photograph in the same lighting conditions, the light will be perfectly balanced. But the moment you move to another lighting condition, you need to re-balance your camera for the new light.
A very similar result can come from using three white coffee filters held in front of the lens to set the custom white balance in your DSLR camera (Digital Single Lens Reflex Camera.) The white coffee filters are not calibrated like the white surface of the Expodisc, but it's very close.
The Expodisc's front surface (seen on the left photo) also has a prism-like glass which spreads the light out evenly across the entire front surface of the disc. This helps get a good reading when taking a white balance reading while doing a custom white balance with your camera. You can't do that with the three white coffee filter trick.
This is just another way to correct the colors in digital photography. It takes some effort to get good photos, but it's worth it.
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