Allows you to use your Zigbee devices without the vendors (Xiaomi/TRADFRI/Hue) bridge/gateway.
It bridges events and allows you to control your Zigbee devices via MQTT. In this way you can integrate your Zigbee devices with whatever smart home infrastructure you are using. Integrates especially well with Home Assistant.
See Supported devices to check wether your device is supported. If it's not listed there support can be added (fairly) easy, see How to support new devices.
The wiki provides you all the information needed to get up and running!