Sensor is open electric curcuit with operational amplifier. After putting measuring element (board) to wet ground on analog output A0 appear voltage inversely proportional to soil conductivity. It is if this base will be completely dry, output voltage will be high. Similary device can be used as flooding sensor. In a case when water apear on the end of sensor board, output voltage go down. 
Other feature of sensor is digital output D0. By potentiometer can be set the switching thresold. For example when water apear, digital output go to down state (as active).
Both outputs an be used with Lan Controller, GSN Controller and all Arduino compatible boards.