From Halopedia, the Halo wiki
A Cylix is a genetic repository used by Forerunners to store and catalogue indexed specimens. Functioning as instrumental tools for the Lifeworkers Conservation Measure, cylixes were mainly used to protect population samples in order to later reseed life in the galaxy after the activation of the Halo Array.
Design and purpose
Cylixes take the form of large slabs with stylized holograms depicting the type of specimen that resides within them. The metallic surface of a cylix has been described as being so cold that feels like one's hand is burning as they touch it. When a cylix is opened, the face that is adorned with the hologram slides apart and the receptacle's occupant is released. While their main purpose was to facilitate the repopulation of the galaxy, in some cases the Forerunners used cylixes as prisons for entities that were deemed to be a threat. It appears that cylixes can either be opened en masse via automated system, or individually by a Reclaimer.
Forerunner Keyships, such as the Anodyne Spirit, carried large numbers of cylixes to be delivered by these vessels to various planets throughout the galaxy during the reseeding process that followed the Halo Array's activation. As of 2559, the Anodyne Spirit still holds what appears to be "millions" of these cylixes within one of its vast interior chambers, a region of the ship that the Covenant referred to as "the clerestory."
Cylixes within Installation 07 contained species such as humans, Jiralhanae, Kig-Yar, Sangheili and Unggoy, in addition to Flood and Xalanyn. The Xalanyn had mysteriously managed to survive the activation of the Halo Array, resulting in the surviving Forerunners deeming them a threat and imprisoning the species within cylixes on Installation 07. One member of the Xalanyn, known as the Harbinger, was released from her cylix by order of War Chief Escharum during the Installation 07 conflict in 2560.
List of appearances