" I wonder what fibre optic cables actually are"
They are cables made from very thin strands of glass (or clear plastic) which transmit data using light instead of electricity, you might have seen the more gimmicky use of them in lamps and Christmas trees.
http://www.canadianwh...ashing/fiberlamp2.jpg
"could I buy one for my PC"
Well, you could, but you'd get no benefit from using fibre optic network cables over traditional network cables unless your computer was located a few miles away from your router.