A vacuum bed or vacrack, is a bondage device designed to hold a victim immobile by removing the air between two sheets of rubber or latex held in a frame (the 'bed') much in the same way as a piece of vacuum packed meat. The victim is equipped with a breathing tube to avoid suffocation.
More expensive vacuum beds use special pumps to expel the air, whilst cheaper ones rely on a vacuum cleaner to provide suction.
Similar devices are also known in the medical field, where it is most often called a vacuum mattress or vacmat. They are typically used in trauma emergencies where it is important to move a person at the same time as having them immobile.
The term vacuum bed is also used in other contexts, such as conveying sheet glass or metal, and in engineering, for removal of dust on a cutting platform - make sure you buy the right thing!