A Role in Kademi grants a specific set of permissions to a certain type of object. You add roles to groups within a context, which is either an organisation or a repository, and that role then only applies within that context.

For example, the Content Author role gives the WRITE_CONTENT permission to content items, ie resources within repositories like web pages and images. If granted with an organisation context that permission is granted to all repositories in the organisation. If granted to  a specific repository then the permissions are only granted to resources inside that repository.

The Administrator is a special role which provides access to all resources no matter what.

Roles can be defined by applications. For example, the e-Learning application has a Learner role which grants recipients access to course and module resources, and allows them to start and complete modules.

More information here on the manage groups screen help  page