Bonjour is a zero config networking thing. In iTunes it allows you to share your music with any computer on your network without setting up any network stuff, you just turn it on and it works.
That is really all it does. But it digs a little deeper into the OS the iTunes needs to so it is a separate service. Pretty much harmless.
Unless it fucks up the install of iTunes.
But it is pretty much Zeroconf done by Apple.