High school chemistry reminder. A salt is a compound made up of a metallic and nonmetallic element ionically bonded together. There are hundreds of different salts.
Molton salt reactor designs work because of the properties of salts. The radioactive fuel is mixed into the Molton salt and the salt acts as a moderator for the reaction. Using molten salt lets you achieve much higher temperatures(and thus higher energy outputs) without having to use a high pressure vessel like in modern light water reactors. There are a couple other properties that remove the possibility of a runaway reaction, and they can develop some unique safety mechanisms like a 'freeze plug' that can be passively triggered to drain the fuel medium into a storage tank to cool down in the event of catastrophic failures of other systems.
