mqtt works topic based. It is for sending and recieving messages. You’ll need a broker to distribute the messages. Every client can subscribe to list of topics, the broker saves that list. If a client sends a new message with the topic foo, the broker recieves it and will try to send it to every subscriber of foo. When some client is offline and comes back up later the broker will still send it to them. Therefore it is great for IoT and smart Home devices