Imagine a town that has only one main industry or type of job, like a town that only makes cars. This town is called a Monotown because it relies only on one thing to make money and keep the people who live there employed.
The problem with Monotowns is that if anything were to happen to the main industry or job, like if the car factory closed down or moved away, then the people who live in the town would lose their jobs and might have to move away to find new work. This can make things really hard for the people and families who live in Monotowns.
So, Monotowns need to be careful and try to find other things to do or make in case something happens to their main industry. This way, they can keep people employed and make sure the town stays strong even if there are changes in the economy or industry.