The Cryostasis Chamber is a machine designed to place a subject into suspended animation by lowering their entire body temperature to absolute zero (-273.15 ˚C, or 0 Kelvin).
HYDRA placed the Winter Soldier in one of these chambers to keep him under control. They would only unfreeze him when they had a mission ready. That way, he could last the decades, and not succumb to age and become wasted. Furthermore, he could not reactivate his lost memories. As soon as the mission was over, HYDRA would return him to this chamber.
After the Duel of Harlem, Emil Blonsky was imprisoned in a Cryostasis Chamber in Alaska. Phil Coulson threatened Grant Ward to post him to guard Blonsky in his cryo-cell on the night shift if his sexual relationship with Melinda May had adverse effects on Coulson's Team.
Coulson delivered Creel in the chamber to Glenn Talbot inside a van. He warned Talbot that the chamber's power had less than an hour before it deactivated because it needed a more stable power source. 
The Winter Soldiers
When Howard Stark managed to either create or obtain several compounds of the Super Soldier Serum, HYDRA sent the brainwashed James "Bucky" Barnes to kill both Howard and his wife, make it look like an accident, and retrieve the serum. After delivering the serum to HYDRA, they used the serum to create several more super soldiers, all of whom were intended to work alongside Barnes and share the mantle of "The Winter Soldier". However, after they managed to overcome their brainwashing and attempted to fight back over their captors, they were kept in cryostasis at the HYDRA Siberian Facility until such a time when they were needed. While they were asleep, HYDRA continued to use Barnes as their master assassin.
As Helmut Zemo manages to temporarily revert Barnes back to his Winter Soldier state, Barne gave up the location of the other super soldiers and reveals his part in the death of Howard and Maria Stark. Zemo then traveled to Siberia and murdered the other super soldiers in their sleep, leaving their dead bodies within the cryostasis chamber.