"Iodine-131 (usually as iodide) is a component of nuclear fallout, and is particularly dangerous owing to the thyroid gland's propensity to concentrate ingested iodine and retain it for periods longer than this isotope's radiological half-life of eight days. For this reason, people at risk of exposure to environmental radioactive iodine (iodine-131) in fallout may be instructed to take non-radioactive potassium iodide tablets... Ingestion of [a] large dose of non-radioactive iodine minimises the uptake of radioactive iodine by the thyroid gland."
This is different than iodine added to salt, or iodine in liquid form for wounds. I believe it is potassium iodine.
It is only taken when the nice person on the radio tells you to do so (you know to tune in from the blaring emergency sirens, tested semiannually). Rather interestingly, the emergency instructions for evacuation state that parents should not go to schools to get their kids, because they'll already be gone: in an emergency, municipal transit buses head to the schools, then take loads of kids to designated areas well upwind of the plant.
All of this info and more is available in a rather well-produced section of the local phone books. Since phone books tend to end up unopened in recycling bins these days, I suspect most newer residents have little idea of what I'm talking about.
https://www.klartext-nahrungsergaenzung.de/wissen/lebensmitt...
[edit]: This one, for the curious, although I get it from Vitamin Shoppe, not direct
https://www.lifeextension.com/vitamins-supplements/item02040...