Okay, imagine you are Snow White and you have a special mirror in your room. This mirror is not like a regular mirror that shows you your reflection. It is a special mirror called a magic mirror.
This magic mirror can talk and it knows everything about the world. When you ask it a question, it will give you an answer. But, here's the cool part, it can only answer one question honestly and truthfully.
So, let's say you stand in front of the mirror and ask, "Mirror, mirror on the wall, who is the fairest of them all?" The magic mirror will think and then give you an answer. It might say, "Snow White, you are the fairest of them all!"
But, if another person, like the Evil Queen, asks the same question, the mirror might give a different answer. It could say, "Evil Queen, you are the fairest of them all!" The mirror's answer will depend on who asks the question and who it thinks is the fairest at that moment.
The magic mirror's job is to show the truth, even if it's not what you want to hear. It can see and know things that we can't and that's what makes it so magical.
So, the magic mirror in Snow White is a special mirror that can talk, knows everything about the world, and can give honest answers to one question. It's a magical and important part of Snow White's story.