The IAC Driver in Audio Midi Setup

Hi,

I'm a newbie in computer music(Mac as well),I have a few questions about midi devices setup.

Firstly,what is the "IAC Driver" in the midi devices of Audio Midi Setup?

In the properties of IAC driver,ports (bus) can be added.But in what condition should the ports be added?In short, how to configure the IAC Drivers?

Thanks in advance.

Cheers,
Edtayin