Potassium Iodide is a salt (inorganic compound with formula KI), that protects the thyroid and glandular system from radiation damage or poisoning. They are also sold as Iodide Pills or Radiation Pills.
After the nuclear reactors in Japan were damaged from the earthquake that hit the region in March 2011, a Potassium Iodide craze was recorded. The rush even reached The States, with fears that winds could blow radiation into the west coast of North America.
Iodide lessens the amount of radiation the human thyroid can absorb. The assimilation of potassium iodide ensures that the thyroid is already full of non-radioactive iodine and has no incentive to take on any more.
The Japanese government handed out large quantities of such supplements to the survivors north of Tokyo.
Potassium Iodide is also available as high-quality liquid iodine supplements.
See also Impact of Japan Nuclear Meltdown on America.