ux4iot is a tool for directly communicating with your IoT devices from your web frontend.
With ux4iot your frontend gets access to Azure IoT Hub's communication primitives without having a custom-built backend middleware translating between IoT Hub and your user interface. No need to design a REST API so that your UI can offer IoT functionality.
The permission logic (which users can perform which actions on which devices) is separated out into a slim backend service that you provide. In this service you focus solely on the permission logic without dealing with any of the communication-related parts - those are handled by ux4iot.
ux4iot is deployed as a Managed Application in your Azure subscription. This means that you have your own dedicated instance of ux4iot and your IoT data does not leave your Azure subscription.