In Angular, component and services are simply classes with decorators that mark their type and provide metadata that tells Angular how to use them. So, metadata is used to decorate a class to configure the expected behavior of the class.

In Angular, metadata is data that provides information about the components, directives, or modules. Metadata is typically defined using decorators such as @Component, @Directive, @Pipe, and @NgModule. This metadata tells Angular how to process and instantiate these constructs.

For example, in a component, metadata includes details such as the template to use, the selector to identify the component in HTML, any inputs or outputs it accepts, and more. Similarly, directives and pipes also have metadata that define their behavior and usage.

When Angular bootstraps an application, it analyzes the metadata of the components, directives, and modules to create the necessary dependency injection structure and to render the application as defined by the metadata.

So, in summary, metadata in Angular provides configuration and structural information that Angular uses to render and manage the application components, directives, and modules.