Cryo chamber

A Cryo-chamber is a plastasteel chamber that allows all UNSC non-essential personnel to be frozen for long periods of time. They are basically large metal pods with form-fitting gel beds where the personnel are frozen until they are needed. An example of this is the only surviving SPARTAN-II super soldier from when the Covenant made a surprising attack on the human military base on the planet Reach. This is the Master Chief, who was stored in a cryo-chamber aboard the ship UNSC Pillar of Autumn and later Tech Chief Thom Shephard along with Tech Officer Sam Marcus, unhatched the Master Chief from the cryosleep chamber at the beginning of Halo: Combat Evolved.

The cryo-chambers are usually stored in their own areas of the ship. Each cryo-chamber will hold an individual for as long as necessary, provided they use a Bronchial Surfactant, and can be ejected prior to an emergency landing, thus saving some crew from harm to be recovered later. Spartan-058, after being severely injured during the fall of Reach, was placed in a cryo-chamber in the UNSC Pillar of Autumn. Prior to the ship's landing on Installation 04, her cryo-chamber was ejected, and she was picked up by the Master Chief at a later date.

Cryo-chambers are monitored by the Ship's AI, and the AI can awaken crew members at a moment's notice. Cryo-computer is designed to change the pharmaceuticals dosage if a individual appears as though they might wake from cryogenic sleep. Cryo wake-up protocols demand that crew check each other to make sure everyone revives from cryo all right. There is usually a medical pod in cryogenics bays for emergency resuscitation situations. People are almost always stored in cryo-pods naked, as covered skin generally reacts badly to the cryo process, leaving skin raw and blistered.

However, in Halo: Combat Evolved, the Master Chief emerges from the cryo-pod completely fitted in his armor, using Spartan training to ignore the pain. His enhanced physiology presumably allows for a quick recovery from superficial pain and injury such as this.

It is unknown whether the Covenant has a similar technology, but it is likely considering the vast distances of interstellar space their ships likely need to travel. Even though Covenant ships are much faster than UNSC ships, Slipspace travel is still not instantaneous.

After the battle of Installation 00, after the Arbiter flies the front half of the UNSC Forward Unto Dawn through the Ark Portal, the back section with the Chief and Cortana remain behind. Cortana sends out a signal, but it will take years for it to reach somewhere. To wait until then, the Chief goes into cryo sleep until someone finds him.

Trivia

 * Cryo-chambers are referred to as "the freezer" in slang, named after a term often called freezer burn which is a pain in joints and muscles caused by the cryo sleep pharmaceuticals trapped in muscles and joints.
 * It it appears that the Cryo-chambers in Halo 3 are slightly different from the original version in Halo: Combat Evolved, this may simply be due to graphics improvement or it could be that the technology has improved (which is plausible due to relativity, less time passes for those on board a ship, although not enough, cryo-chambers are still needed for longer journeys, even when using Slipspace travel).
 * Slipspace Field Pod is the Forerunner equivalent to the Cryo chamber, the Covenant has no known long term storage devices like the Cryo chamber that have been directly observed.
 * While in cryo, humans don't age. Individuals in cryo tubes are given a steady supply of pharmaceuticals that control the frequency and duration of cryo-sleep cycles along with influencing the content of their dreams.