Freeze drying is actually pretty neat

The first step is indeed freezing, basically the same as you would in a regular freezer

But then you take that frozen food, keeping it frozen, and put it in a vacuum chamber.

You might remember from sciences classes in school that different atmospheric temperatures result in water (and other things) freezing or boiling at different temperatures. It's why water boils faster at a higher altitude (and why some packaged foods and recipes have different instructions if you're more than X feet/meters above sea level, the air pressure is lower and so water boils at a lower temperature.

You may also have heard the term sublimation, where a solid turns into a gas without melting into a liquid in-between, like dry ice does, which is solid carbon dioxide, and why it's "dry"

Under a vacuum, ice does the same thing, it turns right into water vapor without melting into water in between.

It actually does this under normal pressure too, but much more slowly. That's actually a lot of what freezer burn is-the water in your food sublimating away into water vapor. And if you've ever left some ice cubes in a freezer for a really long time you might notice they sometimes kind of shrink and get misshapen even though the temperature never got above freezing.

Side note- water actually does kind of a lot of weird stuff when it comes to freezing and melting, in like how given the right conditions, even at normal atmospheric pressure, it can melt or stay liquid well below its freezing temperature, and of course the fact that it expands when frozen.

So the end result is a totally dry, usually pretty shelf-stable product. Because it was frozen, it can retain a lot of it's flavor that might have been cooked off or evaporated with other drying processes.

Some things also take on an interesting texture from the process because all of space in the food that used to be full of water is now full of air. Freeze dried fruits, for example, tend to be really crisp and crumbly sort of like a chip or a cracker, where dehydrated fruit often can be sort of leathery.

And the vacuum process also has effects on some foods besides just drying them out. Skittles, for example, are sort of sealed by their candy shell, so they expand and pop, sort of like popcorn, due to the water inside of them sublimating and expanding until the shell cracks.

If memory serves me, the marshmallows in lucky charms are freeze-dried, which is why their texture is dry and crunchy instead of gooey and fluffy.