Local Area Network. If you want to link a few computer locally together, you need it.
Yes, you can connect to BB with it if your BB modem has Ethernet connection. However, almost all "free" BB modems supplied by ISPs use usb connection, not ethernet connection
if you do not use it, it is better to disable it, you can do it in "bios" which you can get into by press "del" immediatly when you switch on PC