Metadata is a set of data applied to all devices that inherit this Product ID. Metadata is used for data that doesn't change very often (unlike sensor data, which should be defined in Datastreams. In general, it's a key: ""value"" data
First, you, as developer define all the Metadata fields. When device is activated, users can fill in the Metadata
List of defined Metadata will be similar for every device, but values can be different for each device.
An example of Metadata could be a Serial Number of device or Address where device is located.