Okay kiddo, imagine a really big land that has lots of different places and people in it. Now, sometimes there are parts of this land that are really hard to get through, like big mountains or really thick forests. These parts are called natural frontiers.
Now, these natural frontiers are important because they can make it hard for people who want to come in and take over the land. For example, if there is a big mountain range between two lands, it can be very difficult for an army to cross it and invade the other land. So in a way, these natural frontiers help protect the land and the people who live on it.
Sometimes people use natural frontiers as a way to divide the land into different countries or states. For example, a river might be used as a natural frontier to divide one state from another. This way, people can live in their own place with their own rules and be protected by these natural barriers.
So, natural frontiers can be like big walls that protect the land and the people who live on it. They can also help divide the land into different places so everyone can have their own space.