Imagine you have to walk around your neighborhood and deliver letters to all the houses. Some of the streets in your neighborhood are one-way streets, so you can't just walk up and down them. You want to find the shortest possible route that allows you to deliver all the letters and return to where you started.
The Chinese Postman Problem is like that, but much more complicated. Instead of just one neighborhood, you have to find the shortest route that covers all the streets in a whole city or a very large area. Some of the streets have one-way traffic, so you have to navigate around them. And to make it even harder, some streets might have more traffic than others or be narrower or harder to walk on.
The complexity of the Chinese Postman Problem is all about figuring out how hard it is to find the best route. The more streets there are, the more complicated it gets. The same goes for lots of one-way streets or streets that are hard to navigate. Figuring out the best route takes a lot of fancy math and computer algorithms, but sometimes it's just not possible to find the absolute best route without spending way too much time and resources.
To sum it up, the Chinese Postman Problem is trying to find the most efficient way to travel through a network of streets with different conditions, and the complexity of this problem depends on the size and complexity of the network.