IoT is a mechanism to transfer message from one machine to other, or we can say that IoT is a communication between machines.
For an e.g.: -Wristwatches who measure a lot of things and send all the records to your mobile, this machine to machine or device to device communication is IoT.
So, MQTT is the protocol on which it is working. MQTT is MQ telemetry transport which is responsible for transporting message b/w the machine. It’s a protocol which transfers message from one machine to other with very low bandwidth and low strength.