Trying to keep it simple. They all do the same thing. In older, analog systems, 1-11, B727, 737-2, and even some of the pseudo digital systems which are basically analogue with a digital display each tank will contain one compensating probe amongst all the other probes. The compensating probe has an extra capacitor.
A densitometer (American terminology) and a Cadensicon (French terminology) are similar components.
All three attempt, with varying success, to correct for variation in Specific Gravity of the fuel.
The accuracy of a fully serviceable, modern, digital system would astound those used to a simple analogue capacitance bridge system. I know it did me the first time I saw one !