This humidity sensor consists of 2 copper conductors that are located at small distance from each other. The relay switches as soon as the moisture makes a connection that is more or less electrically conductive between the 2 electrodes.
How does the humidity detector works?
When the electrical resistance between the 2 sensors drops below a certain value then the Schmitt trigger (T1 and T2) switches. The RS N1/N2 bistable multivibrator is flipped through C1 so that in point B we have a low voltage and so T3 will close the relay.
The relay is opened when the 10K resistor is connected to point A, not to point B as shown in the schematic. You can use other sensors like LDR or NTC instead of the copper ones so you can use this circuit for detecting light or temperature.