Fin Rot sounds just as it seems. it's a diease that eats away at your goldfish's fins.
It's a Bacterial diease.
Stress and poor water qaulity are the causes of it. So keep keep changing your goldfish's tank. Do not put your gold fish in little bowls you always seen on tv. You can buy a bowl at walmart for the same price of a 10 galloon tank. (even if goldfish need 20 g)