Cryo chamber

A Cryo-chamber is a 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". SPARTAN-117 makes his first appearance in Halo: Combat Evolved inside a cryo-chamber. 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 a ship 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 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 wake-up protocols demand that crew check each other to make sure everyone revives from cryo alright. There is usually a medical pod in cryogenics bays for emergency resuscitations. Many times people are 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.
 * 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).