What is Bonjour?

I have a macbook pro and and iPhone 5, and I want to use messages (on macbook) to make audio calls to my contacts who also have apple devices.

When I open the messages app, look at the drop down to my accounts and then turn off bonjour it won't let me be online for messages.

I have had my macbook for about a year and I never used ichat or imessage before. And I just updated to Mountain Lion a few weeks ago and so far I am very confused.