Here is Carnival's link to how FCCs work. It needs to be updated, as snoozeman says you no longer have to snail mail them in.
What is the Loyalty and Cruise Vacation Program - FAQ's - Carnival Funville
They have also gone from 5 years, to 4 years and now to 2 years. I have been doing group cruises where I cant apply them and have a bunch (4?) of them burning holes in my pocket I will probably have to cancel and get my money back.
OP you asked if you get a good discount . You get no discount using them. They are only good for onboard credit.