They're weapons that use nuclear reactions. It's usually done by fission or a mix of fission and fusion. Let's say you have TnT. Just a lot of explosive. Think a loooooooooooooooooooooooot of explosives.

They're so strong and dangerous that they aren't usually used because of how destructive they are. These weapons can take entire cities, and the damage done can last for many, many years because of the radiation. Even after the initial explosion, the radiation that is caused by these bombs can cause birth defects and internal issues.

They sound reeeeeally cool, but they're not very nice.