An Ancient pressure suit is a protective suit to be worn by a person in an environment they would otherwise be unable to survive such as underwater at extreme depths.
The suits are capable of withstanding extreme pressure and were used on the Mobile drilling platform to make repairs on the outside of the station. They appear to have a Radio in them. (SGA: "Submersion")
The suits, which were invented by the Ancients and have so far only been seen on the mobile drilling platform in the Lantean ocean, were used by the Atlantis expedition to traverse the distance from the station to a crashed Wraith cruiser to try to disable the Self-destruct that a Wraith Queen had set, which when detonated would destroy Atlantis. (SGA: "Submersion")