A decompression chamber is a large container that divers can enter after they have surfaced to help their bodies return to normal atmospheric pressure. It is also called a "recompression chamber" or "hyperbaric oxygen therapy chamber".
When a diver returns to the surface too quickly, he can suffer from the "bends." To prevent this divers would stop at various points as they returned toward the surface to allow their bodies to adjust to the pressure change. If a driver uses a chamber, the diver will adjust in the chamber rather than stop at various depths underwater. This eliminates many of the risks of long decompressions underwater, in cold or dangerous conditions.