In Angular, modules are used to organize the application into logical blocks of functionality. A module is a container for a group of related components, services, directives, pipes, and other modules. Modules help to keep the code organized, maintainable, and easy to scale.

An Angular module is defined using the @NgModule decorator. The decorator takes a metadata object that specifies the module's properties, such as the list of components, directives, pipes, and services that the module exports, the list of modules that the module imports, and the bootstrap component that the application should start with.

Angular applications have at least one module, which is called the root module. The root module is responsible for bootstrapping the application and defining the global services that can be used throughout the application.

Angular modules can also be used to lazy load components and routes, which means that the application loads only the necessary code when it is needed, improving the application's performance and reducing the initial load time.

Overall, modules are an essential part of the Angular framework and provide a powerful way to organize the application's functionality into self-contained, reusable, and maintainable units.