Freeze dried foods are frozen during the process, but not frozen in their finished state. The freeze dryer takes all the moisture out of the food, leaving it shelf stable for up to 25 years when stored appropriately. I'm not sure I can explain the science behind it. The freeze dried food can be rehydrated by adding back in approximately the amount of liquid that was removed. Sometimes, that just means adding it to what your cooking, such as veggies for a soup. There are various ways to rehydrate different foods.
Someone on here said it concentrates the flavor. I second that. Freeze dried fruit is delicious in its freeze dried state without hydration. Almost like a natural candy. But, I ate too much freeze dried pineapple once and it left my mouth raw for a couple days.

The pieces were so much smaller, and it was so easy to just pop one in my mouth after another taking in way to much citric acid at one time.

I'm way too old to be doing stupid stuff like that.