This is 2009, which computer weren't connected to something that doesn't have a DHCP server?
MAC address is the media access control address. The physical hardware address that your "port" or "adapter" has. That printed label on your router or network card that has 00-18-02-66-61-ce or whatever it is.
Normally you don't need to worry about it, it is for the network protocol to detect where to send things to and from. In some case, if you have a network connection that only accept one known adapter (i.e. Cable modem) then you have to spoof that address onto a router so that it can continue to communicate as something it "thinks" is the right device.
It is also useful to spoof this address if you want to access your neighbor's network if he blocks all but his own computers.