A Routemaster is like a big, red bus that you might see driving around the streets of London. It was made a long time ago, in the 1950s, and was designed to be a special type of bus that could carry lots of people and be really easy to get on and off.
The AEC Routemaster has a special feature called a "platform" at the back. This is like a little flat area where people can step on and off as the bus is driving. It's a bit like jumping off a diving board into a swimming pool, except you don't jump as high and the water is a busy street!
Inside, the Routemaster bus has a driver at the front who drives the bus and makes it go where it needs to go. There are also seats for passengers to sit on, as well as some stairs at the back that lead up to a second level of seats.
Because the Routemaster was designed many years ago, it doesn't have some of the fancy features that modern buses have, like air conditioning or TVs. But lots of people really like riding on a Routemaster because it's a part of London's history, and it makes them feel like they're in a classic movie or book.