What are the ports under the Nintendo gamecube for?

OK so one day im moving around/organizing my games and systems and i accidentally drop my gamecube, i catch it but upside down and i see 3 ports, a HI speed port, a serial port 1 and a serial port 2. i tried looking it up but all my results just told me they came with the gamecube, not what they are for. so in other words...can someone tell me what they are for?(i know i probably cant get these attachments anymore, i just wanna know what they are for)