Rusting takes place through a juxtaposition of iron and oxygen that actually constitutes a miniature electric battery, on the atomic scale.
That is, the oxygen molecules are taking electrons away from the iron atoms, and that is exactly what goes on inside a battery: electrons being snatched from one substance by another. Anything that helps electrons to go from the iron atoms to the oxygen molecules will help this process along.
Salt helps because when salt dissolves in water it makes a solution that is a good conductor of electrons. Therefore, salt helps iron to rust by helping to deliver the iron atoms’ electrons to the voracious oxygen molecules.
In the rather complex atom-by-atom mechanism of rusting, salt also helps to conduct charged iron atoms (ions) to where they need to go. Moreover, the chloride in the salt, which is sodium chloride, has a separate effect on the iron.
But that’s all a bit more than we want to get into. Trust me. Just don’t drive your car in salt water.