Okay kiddo, have you ever seen a bridge? You know, that thing we cross when we need to go over water or something else? A natural arch is kind of like a bridge. But instead of being built by humans, it's made from rocks and formed by nature over a very long time.
Imagine that you have a pile of sand, and you put your hand underneath it and push up. When you take your hand away, the sand is in the shape of an arch. That's basically how a natural arch forms, except with big rocks instead of sand.
Sometimes, the wind or water will wear away the surrounding rocks so that only the arch is left standing. Natural arches can be found all over the world in places like national parks, deserts, and even near the ocean. They can be really big or really small, but they are always a pretty cool thing to see!