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](https://www.home-assistant.io/).
See [Supported devices](https://github.com/Koenkk/zigbee2mqtt/wiki/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](https://github.com/Koenkk/zigbee2mqtt/wiki/How-to-support-new-devices).