By potassium tablets, there are two possibilities. One is a potassium supliment to help replace the least common component in electrolytes. We get plenty of salt normally.

More likely, it is those things that the DoE thinks might make a difference if you arewithin ten miles from a nuclear reactor. Potassium iodide work by adding a degree of protection to your thyroid if you are in a irradiated area, but you have to be taking them before exposure for them to be any good. The "anti nuke" pills really aren't worth much if you ask the other nations with nuclear power and decent medicine (IIRC, niether the Japanese, Brits nor the French issue them to thier civilian population, and they all use percentagely more nuclear reactors than we do), and definantly aren't worth the cost and bulk in a field first aid kit.