What does potassium iodide do for you in a nuclear accident?

A nuclear event, such as an accident or war, is likely to produce a lot of radioactive iodine. The thyroid gland concentrates iodine, so any iodine that gets into a person's body is likely to end up there. The potassium iodide tablets provide more iodine than the thyroid can use, so the radioactive iodine is not so likely to be retained in the body. So the potassium iodide reduces exposure to radioactivity. It protects only the thyroid, and it only protects that against radioactive iodine, but it is better than nothing.