An MQTT Tester app is a software tool used for testing MQTT-based applications. It provides a graphical user interface (GUI) that allows users to interact with an MQTT broker and send/receive MQTT messages. With an MQTT app tester, users can specify the topic, message payload, and other parameters for the MQTT message, and the tester will send the message to the MQTT broker and receive any responses from the broker. MQTT Tester app can also simulate a large number of MQTT clients connecting to the broker and sending messages simultaneously, helping to identify any performance bottlenecks or scalability issues in the application. Overall, an MQTT Tester app is a valuable tool for developers and testers who are working with MQTT-based applications.