It is not an antibiotic, it is a medicine that inhibits the growth of cocci which is an intestinal parasite. You can find more info by doing a search on this. Alot has been written.
It's medicated with Amprolium which is not an antibiotic but a coccistat ( forgive the spelling) to help with cocci ( also forgive the spelling.) It helps chicks build an immunity so that when they are exposed to it, they won't get sick. That's basically laymen terms. Some folks use medicated feed, and some don't. It's a matter of preference.
You can google Amprolium and will be able to find a lot of info on it.