The center of each MQTT project is the MQTT server, sometimes referred to as the MQTT broker. It is the control center to which each device in the MQTT network needs to connect, where all of the MQTT messages are sent to, and where devices can subscribe to the topics that they are interested in. It handles the topic filtering, so it knows which devices are interested in which topics, and forwards the messages accordingly. It also keeps track of messages that could not be delivered yet; for example, because a subscribed device went offline. The server will try again, until the device goes back online and the message can be delivered.
Because MQTT is an open protocol, you can freely switch between MQTT servers. In contrast to proprietary solutions, where you cannot easily switch servers, MQTT makes this very easy. In most cases, you just...