A billboard bicycle is like a big sign that somebody rides around on a bicycle. The person who is riding the bike is pulling a big rectangle or square that has a message or an advertisement on it.
So imagine that you're at a big event, like a parade or a festival. You see people riding bikes, but on top of the bikes they have a big sign or a banner that they're pulling along with them. The sign could be really big, maybe big enough for you to see it even if you're far away.
The point of the billboard bicycle is to get people's attention and show them a message or an advertisement. It's like a moving billboard, because the bike is moving around and going to different places, so more people can see it.
It takes a lot of work to ride a bike that has a big heavy sign attached to it. But some people like it because they get to exercise and get paid at the same time. And for the people who are looking at the billboard bicycle, it's a fun and interesting way to see different messages and advertisements.