The MessageProperty Structured Data Type is defined under the GeneXusMessagingQueue.SimpleQueue module and represents the attributes of a message (Key and Value pairs) that are sent as part of the message, or returned when a message is received.
The attributes depend on the provider (Azure Queue or AWS SQS), both can return different metadata information after sending a message to the Queue.
In the case of AWS SQS, it lets you include structured metadata with messages using message attributes. For more information see this link.