The most simple answer about what a superheater is: Copper tubes that take the steam from the boiler and the tubes makes a double trip down the flus of the engine which raises the temperature of the steam higher than the temperature of the steam in the boiler.
On engines with front end throttles the route of the steam is a little different than engines with throttles inside the boiler but the heating of the steam is the same way.
Superheated steam expands more than saturated steam and therefore the engine uses less steam and consequently less water and coal.