Freeze-drying is a method of preservation that makes use of the physical principle known as sublimation.
It's a water removal process which involves freezing the product, lowering the pressure to allow the ice (frozen water) in the product to change directly to a vapor (sublimate).
This is different to dehydration by most conventional methods that use heat to evaporate the water.
Freeze-drying is one of the most desirable way of preserving food because at low temperatures chemical reactions take place very slowly and pathogens have difficulty surviving.