The kidneys are a pair of bean-shaped organs located in the posterior part of the abdomen, one on each side of the spine. They filter waste from the blood (producing urine) and fulfil a number of other complex biological functions.
The kidneys are sensitive organs and care must be taken that they don't get undercooled (e.g. by wearing a kidney belt when riding a motorbike), and that they don't get injured by a blow to the lower back, for example in sports or in BDSM play.
In spanking, it is very important to never strike anywhere near the kidney area. The spanking should either be restricted to the fleshy part of the buttocks, further down to the sit spots, or if this is not enough it may be extended to the back of the thighs, but never further up to the lower back.
In judicial corporal punishment, e.g. in Malaysia and Singapore, the delinquent is given a padded cover for the kidney area to wear, so this area is protected from being hit by a misdirected stroke of the cane. Some BDSM practitioners recommend to wear a kidney belt for safety whenever an implement is used which is difficult to control, such as a single-tail whip.