Cryo trays (properly styled cryogenic trays) are long mechanical cargo containers each full of 100,000 cryo pods, pods full of living people frozen at extremely low temperatures using cryogenic technology.
The best known cryo trays and the ones usually referred to in context were those used by the Mothership Fleet during the Exodus to Hiigara. Stored for as many years as twelve these people are men and women of the Kushan people who have bought, earned, or somehow else gotten a ticket for the first journey to Hiigara. These people, who were thought to simply be extraordinarily rich or adventurous turned out ironically to be the only remnants of the Kushan people other than the actual Mothership Fleet's crew by the end of the Homeworld War.
Cryogenic technology was first developed by observing and testing with various animals on Kharak's surface that burrowed into the ground during the hot summers and hibernated. By analyzing certain patterns in the way that they slept and their metabolism changed during these hot summers it was easy enough for Kushan engineers, aided by technology provided from Khar-Toba to reverse the process for freezing.
The cryo pods in question were first tested using small vessels with one-man crews, sent out to the edge of the Kharak system and then programmed by autopilot to turn around and head back while the pilot was in suspended animation. The tests proved effective, with surprising results suggesting that cryosleep was in many ways no more different than real sleep, simply that it required less energy.
With the technology successfully tested the first volunteers were frozen about 12 years before the launch of the Mothership. These adventurous, usually very young (although the process was restricted to only those legally of adult age and below the age of senior citizenship) pioneers in the field were then transported to underground facilities where they and additional volunteers overtime were stored until finally the Mothership was prepped for launch and the cryo pods were transferred to the orbital cryo trays.
These cryo trays eventually proved to be the last and only survivors of the savage Taiidan attack on Kharak shortly after the Mothership's first hyperdrive test. Though the Taiidan fleet immediately scoured the surface of Kharak with their deadly atmospheric deprivation missiles the cryo trays were for the most part left to a few vessels which remained to finish the job, figuring that a few defenseless containers of frozen people wouldn't be much of an immediate threat.
Much to the surprise of those involved however, the Mothership Fleet arrived just in time to rescue the cryo trays and save their people from extinction. Once the frigates left to finish off the trays were captured or destroyed the cryo trays themselves were salvaged and brought back to the Mothership for docking. Once the cyro trays were safely stowed aboard, the Mothership left Kharak behind, forever.
Those who survived the attack on Kharak were known as Sleepers, based on the fact that they had been asleep when they were rescued. Many sleepers were awoken during the journey and given immediate emergency training to replenish men killed by the Taiidan during the war. Those that awoke, and had not had time to come to terms with the fact that all their friends and family were dead, immediately became zealous patriots devoted only to one thing: to destroy those who had destroyed all they had known. Though most in time came to terms with their mix of luck and misfortune, more than a few of these sleepers became bounty hunters, assassins, and pilot aces who spent their lives hunting down and killing Taiidan Imperials.
Two of the most famous examples of this are in Iifrit Tambuur, the lone survivor of his entire kiith and pilots of Somtaaw Mimic-class Infiltration Crafts, both of whom came to place the individual deaths of Taiidan Imperialists over their own survival. In either case, though the cryo trays in the end were more than a luxury, the survival of their inhabitants continued to haunt history even after the war that had so nearly destroyed the Kushan people was over. For the sleepers, the cryo trays were a mixed blessing.