Monorepos are a type of source code repository that holds all the code for a single project in one repository. This means that instead of having multiple repositories for different components of the same project, all the code is stored in one place. This makes it easier to manage and track changes across all parts of the project, as well as to ensure that all the components of the project are kept up-to-date. Monorepos are popular among JavaScript developers due to their scalability and flexibility.