Eclipse Mosquitto is an open-source MQTT message broker designed for lightweight, efficient communication between devices in IoT and M2M applications. It implements MQTT protocol versions 5.0, 3.1.1, and 3.1, supporting a publish/subscribe messaging model ideal for low-power sensors and embedded systems. Written in C, Mosquitto is highly portable and can run on various platforms, including Linux, Windows, macOS, and embedded devices like Raspberry Pi. Key features include support for SSL/TLS encryption, bridge connections to other MQTT brokers, and tools like mosquitto_pub and mosquitto_sub for testing and integration.