Well, a long long time ago, people used to believe in magic and things that were not real (kind of like fairy tales). They believed that there were people who could use magic to do bad things (like make people sick or hurt them). These people were called witches.
The word "witch" comes from an old English word "wicce" which means "wise woman". You see, back then, people thought that witches had special knowledge about herbs and medicine, and that they could use this knowledge to do good things like heal people. But then some people started to believe that witches were actually doing bad things with their magic, like using spells to hurt people, and so the idea of a good witch kind of faded away.
Nowadays, we don't really believe in witches or magic anymore (at least most people don't). We have science and medicine to explain how things work and how to make people better when they are sick. But the word "witch" is still used sometimes, mostly in stories and movies, to talk about a person who uses magic to do good or bad things.