Outbound message in salesforce is a kind of action that sends the information to an end-point (External Service) in the form of XML (Extensible Markup Language). It used to send object field data to your third-party application to perform the specific task as per requirement. It also played a very big role in the Integration of salesforce with any other application. You can send the outbound message using workflow, entitlement processes or approval processes.
The best thing about the outbound message is, you do not need to write any apex code you can create it by simple point & Click. When you send any message to an end-point than it will get the information of the data in the fields which we are sending in the outbound message in the form of XML. You can use RequestBin.com which will provide you the end-point to use and you can get the XML there.