A trailer bus is like a regular bus, but it also has a really big "backpack" that it can tow behind it. Think of it like your backpack that you take to school, but much much bigger! The trailer part of the bus is where more people can sit and ride along with the driver, just like on the regular part of the bus.
The trailer part doesn't have its own engine, so it needs the regular part of the bus to pull it along. It's kind of like when you ride your bicycle and your friend holds onto the back of your bike to help you go faster.
Trailer buses are great for carrying more passengers at once, especially if there are a lot of people going to the same place. They're often used by public transportation companies or for tourist buses.
Overall, a trailer bus is just a bigger bus with a special "backpack" that can carry more people, but it needs the help of the regular part of the bus to move around.