Soil Moisture Sensor Arduino compatible analog and digital output kit contains the soil sensor electrode, and the sensor/trigger board. The power supply should be chosen accordingly to the connected microcontroller, because the output voltage is about the supply voltage. It means, depending on your microcontroller, the power supply should be for example 5V for an Arduino Uno or 3.3V for an Arduino Due or Zero.

Please see the schematic; it shows the trigger circuit which is pretty much the same for all similar-looking sensor boards. Only the analog output is not shown on the drawing, but the analog output is just the straight signal from the sensor electrodes.