Metadata are data that are associated with the processing of the message, on top of the processed input and output files.
For example, metadata can be the date and time when the message was received, the channel that processed the message, information about the sender of the message, ... The complete list of system metadata can be found here.
Metadata is useful to customize your output messages or notifications. See below some usage examples.
It is also possible that you define your own metadata. See Using or changing metadata