A role is a collection of permissions defined for the whole system that you can assign to specific users in specific contexts.
For example, you may have a Role called "Teacher" that is set up to allow teachers to do certain things (and not others). Once this role exists, you can assign it to someone in a course to make them a "Teacher" for that course. You could also assign the role to a user in the course category to make them a "Teacher" for all the courses under that category, or assign the role to a user just in a single forum, giving that user those capabilities just in that forum.
A role must have a name. If you need to name the role for multiple languages you can use multilang syntax if you wish, such as
<span lang="en">Teacher</span> <span lang="es_es">Profesor</span>If you do this make sure the setting to "filter strings" is on for your installation.
The shortname is necessary for other plugins in Moodle that may need to refer to your Roles (eg when uploading users from a file or setting enrolments via an enrolment plugin).
The description is simply to describe the role in your own words, so that everyone has a common understanding about the role.
See also Contexts, Permissions, Assign Roles and Overrides.