Detailed Description
What is a Unit of Desiccant?
A unit of desiccant is defined as "the amount of desiccant that will adsorb at least 3g of water vapor at 20% relative humidity and at least 6g of water at 40% relative humidity at 77°F (25°C)."
If you're really interested, MIL-D-3464E is the source document that outlines the generally accepted method for determining the units of desiccant required (calculation requires the size and the type of the container and the type of desiccant to be used).