A desiccant is a drying agent that is used to absorb moisture from the air inside moisture barrier bags. A desiccant absorbs moisture vapor (humidity) from the air left inside the barrier bag after it has been sealed. Any moisture that penetrates the bag will also be absorbed. A desiccant remains dry to the touch even when it is fully saturated with moisturevapor. Desiccant is sold by the “Unit” or fractional Unit, or in grams. One unit of desiccant will absorb a specific amount of moisture.

To be used most effectively, desiccants should be used within a closed/sealed moisture barrier, or a rigid/sealed container. Humidity indicating cards are generally placed inside the container to show the humidity level and indicate when the desiccant needs to be replaced.

Uses of desiccant include the following: drying flowers, packaging and preserving of food, dried herbs and herb extracts during transportation and storage.

In other words:
A desiccant is a hygroscopic material (it will absorb water from the air). Desiccant bags are used to completely dry the air within a package or container during shipment or storage. The disposable packets are dust tight and will protect enclosures for many months when used within an adequate moisture barrier. It can also be used to dry out herbal preparations or extracts. One 35 gram unit of Clay desiccant can absorb more than 10 grams of water.