Allows you to use your Zigbee devices without the vendors (Xiaomi/TRADFRI/Hue) bridge/gateway.
It bridges the events sent from the sensors and switches to MQTT. In this way you can integrate your Zigbee devices with whatever smart home infrastructure you are using. Integrates especially well with home-assistant.io.
See Supported devices to check wether your device is supported. If it's not listed there support can be added (fairly) easy. Feel free to create an issue in this case.
The wiki provides you all the information needed to get up and running!