I’m not an expert, but it’s always been my understanding that fallout is ordinary dust and smoke particles that were ejected high into the stratosphere by the denotation of a nuclear weapon and then those tiny particles become contaminated with extremely high levels of radioactivity. As the jet stream carries these aerosolized radioactive particles around, they can be captured by clouds and precipitate out as radioactive rain that can contaminate bodies of water and the ground. Not sure if this is 100% accurate though.