It is a basic bridge that is similar to a Wheatstone bridge.
This circuit is basically 2 voltage dividers in parallel: resistors R1/R2 from one voltage divider, while capacitors C1/C2 from the other.
An oscillator operating in the 100 kHz region drives the bridge and is its voltage source.
The output voltage across terminals A and B is zero as long as the ratio of the resistance R1/R2 is equal to the ratio Xc1/Xc2.
resistance-Capacitance bridge circuit