QoS stands for Quality of Service. It is basically a standard for reliability and the successful delivery of each message in MQTT. It makes sure that our messages reach the destination even when a connection is unreliable, and overcomes the issue of data loss. There are three types of QoS as follows:

• QoS=0: It just promises to send the message only, once but doesn't ensure its delivery and any retry in case of failure.
• QoS=1: It promises that the message will be received by the recipient at least once. Until the sender receives acknowledgement from the recipient, it will keep on sending the message, which can result in duplicate messages.
• QoS=2: It promises that message will be received by a recipient only once. It will make sure that there is no data loss or duplication.