An antibiotic is something which kills bacteria, or stops it from growing/reproducing. There are many different types of antibiotic. They’re usually small compounds and they’re made from carbon, hydrogen, oxygen, nitrogen, maybe sulphur – the usual things that small molecules are made of.
They work either by killing the bacteria directly by breaking the bacteria membrane or cell wall or stopping vital enzymes from working – or by a more complicated process of targeting the reproductive parts of the bacterial machinery. This doesn’t kill the bacteria directly, but stops the bacteria from multiplying and allows your immune system to finish the bacteria off.
Good answer from Jack. we still need our own immune system to finish the job!
But antibiotics are made from the same stuff that everything else is… atoms! mostly carbon, with a pentagonal ring, and a square in it. The square shape is the important bit because this breaks open it binds to some enzymes in the bacteria and stops them working.