They threatened to report this to a credit rating agency. At any given time it's probably not a big deal assuming 1) you don't need to replace your car or sign a lease or get a personal loan around that time and 2) you know that you can dispute negative marks on your credit report and you know how to do that.
I didn't know how to do that until I had my identity stolen and I'm definitely more credit savvy than most from my cc churning days