The MQ-135 gas sensor module has four pins (standard male headers). The air quality information is readable via the AO pin. Since AO is a varying voltage, this pin should be connected to one analog input of a microcontroller.
The DO pin is an open-collector digital output (but with a pull-up resistor onboard) that becomes low when the detection level exceeds a predefined level. The detection threshold can be tailored through the small trimpot soldered on the bottom of the module.
Every time your sensor gets introduced to that gas at predefined concentration, DO will go low (0V) else will remain high (5V). Note that the green LED (OUT_LED) should also light up when gas is observed.
When the sensor is powered up for gas detection, it needs around 60-120 seconds to settle, because the heater will need that much time to heat the sensor up. Always use a separate power source for gas sensors and do not power up from Arduino.
MQ-135 gas sensor is suitable for detecting (or measuring) of NH3, NOx, alcohol, Benzene, smoke, CO2, etc.