Đồng đăng is a small town in the northern part of Vietnam that is located next to the border with China. Think of it as a special town where two countries meet and people can cross the border easily. You know how when you go to school, you might have to cross the street to get there? It's like that, but instead of crossing the street, you're crossing a line between two countries.
People from both Vietnam and China can go to Đồng đăng to do things like work, shop, or visit family and friends. Just like when you go to a different town or place, there are different rules, traditions, and people there. When people from different countries come to Đồng đăng, they have to follow the rules and laws of both countries, which can be tricky sometimes.
But Đồng đăng is also a really cool place because you can learn about both Vietnamese and Chinese culture. You might see different types of food, clothes, and buildings that are unique to each country. And who knows, maybe one day you will get the chance to visit and see this special town for yourself!