This MQ137 module is a sensor board designed to detect NH3 Ammonia gas along with other gases. Sensitive material of MQ137 gas sensor is SnO2, which with lower conductivity (high resistance) in clean air. When the target sensitive gas exist, the sensor's conductivity is higher along with the gas concentration rising. The board comes with basic circuitry to change the conductivity or resistance to voltage and output it to header pins, DO and AO.
The figure above shows the connection of MQ sensor pin to Arduino Uno
MQ137 gas sensor has high sensitity to Ammonia, also to other organic amine. The sensor could be used to detect different gas which contains Ammonia, it is suitable for different application.