Cryosleep is "sleeping" or "hibernating" for long periods of time in a controlled environment. While cryosleeping, or "in cryo", a person does not age, does not dream, and does not need food or water. Technologies like cryosleep are licensed by groups like the RDA to keep humans alive and well for long periods of time.
It is invaluable for long space voyages and for giving the Avatar's grown by RDA time to mature with their Avatar Drivers. Waking from cryosleep is described as being quite disorienting and highly uncomfortable, and the longer one is in cryosleep, the worse these side effects become. Some humans waking from cryosleep may experience nausea and extreme weakness, which the examinations caretakers perform on waking passengers on board the ISV.
Cryosleep belongs to a sub-branch of cryonics, or the act of perserving living organic tissue in a frozen state. It has existed well before the 22nd Century, but the actual technology of cryosleep, that has kept living individuals and animals alive after "thawing" them out was first invented in the United Kingdom in 2032. It was later improved over the years by a medical company in the late 21st Century in the United States.
Cryosleep is not actually sleeping. It is freezing a person and maintaining his or her body at a very low temperature until the end of the "sleep", when the humans are gradually re-warmed and thawed out. The problem of irreparable cell damage caused by the formation of intracellular ice crystals was solved by using low doses of microwave radiation to jostle the water molecules as the temperature drops, which eliminates the formation of any ice crystals. The failure rate of this process is less than 1%. However, passengers and their heirs release the RDA for any liability as a condition of their employment.
A Cryovault is the section built within the long stem of the Interstellar Vehicle. It's purpose is to house a good number of 200 RDA personnel. A look inside of the cryovault, one can automatically view that there is no floors to stand upon as there is no artificial gravity. Everyone has their own "pod" to sleep in, but each pod are built together in pairs. The lockers are placed for good line of sight and access for the cryo unit occupants, but this actually orients them counterintuitively (perpendicular) to the space's main walls.