There is a trade-off inherent to QoS levels. QoS is the MQTT terminology for message delivery reliability requirements. It is a common term in networking, and we will review how it is implemented specifically for MQTT.
The trade-off is between bandwidth used and reliability of message delivery. More back and forth messaging is required to ensure higher reliability, which increases bandwidth requirements for the same message packet. MQTT gives you some options to choose between.
QoS is set separately for both the ends of the communication. There is a QoS that is set for the publisher to broker messaging connection and a separate QoS that is set for the broker to subscriber connection. Often, QoS is the same for both, but not always.