Potassium nitrate is best known as the oxidizing agent in old fashioned "black powder" gunpowder, which is 75% potassium nitrate by weight. The other ingredients are 15% charcoal, and 10% sulfur.
But potassium nitrate is also used in toothpastes to make teeth less sensitive to pain. As gums recede and the tooth root dentin becomes exposed to hot or cold foods, the teeth can become hypersensitive, resulting in pain. Potassium nitrate interferes with the transmission of pain signals in the nerves of the teeth, after a few weeks of brushing with a toothpaste containing it.
Potassium nitrate is also found in pills for backache and joint pains.
It also makes a decent plant fertilizer, providing nitrogen and potassium, but no phosphorus. As an oxidizer, it is used in stump removers, to hasten the decay of tree stumps.
Potassium nitrate is used as a preservative in some salted meats. As with nitrites, it helps to preserve the color of the meat.