The Australasian realm is a big area in the world that includes a lot of cool places that are south of the equator. Imagine putting on a big hat and looking down at the world from a really high place. The Australasian realm is everything you see that is below the bottom line of your hat.
This realm includes places like Australia and New Zealand, which are countries that lots of people visit for vacation. It also includes places like Papua New Guinea, which is a big island that lots of people have never heard of before.
All of these places in the Australasian realm are special in their own way. They have different customs, languages, and animals. But they also share some things in common, like the fact that they're all part of the same big area of the world.
So, the Australasian realm is basically a big area in the southern part of the world that includes lots of different countries and cultures.