It saturates your thyroid with iodine, to prevent uptake of radioactive iodine. The best info resource I've found is www.ki4u.com.
Potassium Iodide (KI) is approximately 76.5% iodine.
I have taken Kelp tablets for about 25 years. If it were toxic... my doctor would have found it by now. I have bloodwork done regularly. Like every 6 months at least. Kelp tablets are 150% ... and yes, they are iodine, too, from North Atlantic Kelp!