As per Wikipedia:
The St. Andrew’s Cross, Crux ssata, X-cross, X-frame or saltire cross is a common piece of equipment in BDSM dungeons. It typically provides restraining points for ankles, wrists, and waist. When secured to a saltire, the subject is restrained in a spreadeagle position.The St. Andrew’s Cross and the spanking bench are the most common pieces of BDSM furniture. Saltires are versatile and easy to manufacture. They are usually firmly attached to a wall, and some are attached to a central hinge, allowing the subject to be spun and inverted.
The submissive may be attached to the cross with either the back or front facing the cross. Being restrained facing the cross is the position often used for whipping. Back to the cross is usually more of a sexual bondage position or used for sexual teasing.
Freestanding saltires have a frame which supports the cross. Some homemade freestanding crosses have a tendency to be unstable; a writhing subject can sometimes turn them over, so great caution needs to be exercised in their use. Some freestanding crosses include footrests that greatly improve safety and stability.
The name comes from the Crux decussata (diagonal cross) that Saint Andrew is said to have been martyred on.