Definition - What does Service Broker mean?
Service Broker is a feature of SQL Server that monitors the completion of tasks, usually command messages, between two different applications in the database engine. It is responsible for the safe delivery of messages from one end to another. If a target application is temporarily shut down or is not responding, Service Broker stores the messages until they are ready for delivery.
Techopedia explains Service Broker
When two applications (within or outside of SQL Server) communicate, neither can access the technical details at the opposite end. It is the job of Service Broker to protect sensitive messages and reliably deliver them to the designated location. Service Broker is highly integrated and provides a simple Transact-SQL interface for sending and receiving messages, combined with a set of strong guarantees for message delivery and processing. The initiator program (one that starts the conversation) sends the message to the Service Broker along with the target application (receiver's) address. The target application, after receiving the message, sends an acknowledgement or response message which indicates the successful delivery of to the indicator application.