The turnbuckle is used frequently to tighten shafts and cables in many applications. Essentially a long nut, allowing two threaded parts to come in on either sides. The threads are cut such that turning the nut brings the two threaded parts closer or further apart.
In professional wrestling, it refers to the ends of the ring ropes, forming the four corners. There's probably an actually turnbuckle there, underneath the padding. The whole padded thing is referred to as the turnbuckle.
Frequently, wrestlers whip their opponents into the turnbuckle, theoretically causing pain. In the old days, heels (bad guys) would remove the padding from the turnbuckle to maximize damage. 90% of the time, the babyface (good guy) is able to utilize the now nude turnbuckle himself, giving the heel what he(she) deserves.
Also, the padding was a favorite meal of George "The Animal" Steele.