RACK is also an abbreviation for Risk Aware Consensual Kink
The rack is a torture device. It consists of a rectangular, usually wooden, frame, with a roller at one, or both, ends. The victim's feet are manacled to one roller, and the wrists are chained to the other. Gradually, one or both rollers are gradually turned, using a handle, with a ratchet causing a stepwise increase the tension on the chains, which induces pain as the victim is slowly stretched.
In past times, this was a gruesome torture device, often used to the point where joints would be pulled apart with excruciating agony.
Modern versions of racks are available for BDSM play. They are designed to put the victim's body under tension, instilling a sense of helplessness and vulnerability.