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](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. Feel free to create an [issue](https://github.com/Koenkk/zigbee2mqtt/issues) in this case.