According to Wikipedia "Freeze drying - technically known as lyophilisation, lyophilization, or cryodesiccation - is a dehydration process typically used to preserve a perishable material or make the material more convenient for transport. Freeze drying works by freezing the material and then reducing the surrounding pressure to allow the frozen water in the material to sublimate directly from the solid phase to the gas phase."
Here is another good explanation from Livestrong: "Freeze-drying removes the moisture from foods, decreasing their volume and weight while preserving their nutritional value. This gives foods a much longer shelf life and increases the ways they can be used."
And because we'd never just believe a company that claims to be nutritious, delicious and easy here is some science:
Research conducted by Sheffield Hallam University in 2013, which measured the nutrients in freeze dried, chilled and fresh strawberries, found that refrigerated fruit retained only 20% of their antioxidants. In contrast, freeze drying had no significant impact on nutrient content. The freeze dried strawberries retained all their vitamin C and polyphenols and 92% of their antioxidants.