A moisture sensor is generally composed of two parts, one is an actual sensing module and the one is a small controller board.

Actual Soil Moisture Sensor: dipped in the soil and does the actual sensing, it is two-wired out the sensor, this out is connected to the sensor control board. Need to connect the 2 header pins (+ and -) to the soil moisture sensor control board as shown by the 2 blue wires above.

Soil Moisture Sensor Control Board: This board takes two-wire inputs from the Soil Moisture Sensor and converts them into digital and analog outputs. It has four pins on the output side pins has 4 pins on the other side labeled VCC, GND, D0, and A0.

(https://www.hackster.io/amruthp/soil-moisture-sensor-arduino-tutorial-ffcb5b)