It's a form of suspended animation. It essentially halts the life processes of an individual so they can be placed into storage and revived later.

[Or not revived later.]

Cryofreeze does this by, well, freezing them. I don't know the exact processes used to do this, but they've apparently refined it to the point where it's not fatal. Diabolin criminals would routinely be placed into cryofreeze, and then some other generation of Diabolin would inevitably revive them for their own purposes.