Device twins store device-related information that:
Device and back ends can use to synchronize device conditions and configuration.
The solution back end can use to query and target long-running operations.
Desired properties. Used along with reported properties to synchronize device configuration or conditions. The solution back end can set desired properties, and the device app can read them. The device app can also receive notifications of changes in the desired properties.
Reference:
https://docs.microsoft.com/en-us/azure/iot-hub/iot-hub-devguide-device-twins