The 'Bluetooth Sensor Hub' is an internal ability of some Mi Home smart devices. It allows all Bluetooth sub-devices to access the Internet assuming that they support standard Mi Bluetooth profiles.
Simply place a device supporting standard Mi Bluetooth profiles near a Sensor Hub and it will be able to access the Internet automatically, allowing you to view its status from your phone remotely, as well as to set up automatic, smart connections.
For example: placing a Mi Temperature and Humidity Monitor near a Mi Smart Camera will allow the Temperature and Humidity Monitor to access the Internet automatically through the Camera's Bluetooth Sensor Hub ability, allowing it to transmit current temperature and humidity status updates.