iMessage has a Bonjour option but there isn't any explanation on what this all is about. So what can I do with this?
When Bonjour is enabled in iMessage it allows you to chat with other users on the same network who also have Bonjour enabled. As described in Bonjour For Developers:
This was developed some years ago to enable easy configuration of printers and other network services without needing to know ports, addresses, etc. iMessage simply ties into this service.
Jabber is a messaging protocol. It underlies several protocols such as AIM and Google Chat.