Volumetrics is basically a plan that stresses foods with low energy density, meaning that they have a low number of calories for a high volume. This means you can eat more volume for the same amount of calories, which will in theory keep you fuller, longer.
For example, if you have 50 calories to work with, you can eat 8-10 cups of lettuce, depending on the varieties you're choosing. That's a low-energy-dense food. On the other side of the spectrum, you'd have something like full fat cheese...you'd get half an ounce, or about the size of 2 dice.
You can read more here:
http://diet.lovetoknow.com/wiki/Volumetrics_Diet