What is Issuer secret?? What should be its input.??

I am using PersistentQueueListener from SDK to receive messages from Service Bus. While I run the PersistentQueueListener Project it expects few imputs like

1. Service NameSpace

2. Issuer name

3. Issuer secret.

I don't have a clue of Issuer secret here. and what should be given here. I know the issuer name from "developer resources" in CRM application.