A role is a collection of permissions defined for the whole system that you can assign to specific users in specific contexts. The combination of roles and context define a specific user's ability to do something on any page. The most common examples are the roles of student and teacher in the context of a course.
For your information a number of roles exist in Moodle. These roles can be customised at site administration level and you will likely only encounter teacher and student type roles (in bold) because this is all you will need.
In this introduction to roles we will share with you some basics of teacher view.