It's a sulfonamide drug, full name sulfadimethoxine - not an antibiotic of the kind that usually comes to mind like penicillin. It works better than the standard antibiotics for things like coccidia. Did the vet discuss possible side effects with you? That is the downside of using that class of drug - make sure that your cat stays well hydrated, as has been mentioned it can cause the urine to become concentrated. It's not very likely to happen, but if your cat develops sore looking eyes or pale gums, phone the vet immediately- don't worry unduly though (all drugs have potential side effects), just something to keep an eye on.
I hope your kitty feels better soon