The containment vessel was a device that encapsulates its contents in a layer of subspace. The Ancients were using this technology during the height of their war with the Wraith. Because the substance contained is stored in subspace, an object of virtually any manageable size can be encased within. The only thus far encountered containment device was modified to emit a specific type of radiation meant to attract a non-corporeal energy-consuming being.
Only weeks after the Atlantis expedition had taken up residence in Atlantis, one of these devices was discovered, and its contents let loose in the city. In the device was an energy-consuming non-corporeal being, studied by the Ancients during their research on ascension. However, the consuming of energy was hazardous to human life, and prolonged direct contact with the being was lethal. Eventually, the creature was lured through the Stargate to another planet. (SGA: "Hide and Seek")
Behind the scenes[]
This appears to be the same device as the Dimension observer, albeit one with a different purpose, that of which allowed people to see Interdimensional creatures. Which was featured in the Stargate SG-1 episode "Sight Unseen".
It is possible that the Mask of Anubis utilized a similar technology to this.