The student representatives (Fachschaft) are basically the entirety of all students on a course. In normal usage, however, the student representatives mean the student committee (Fachschaftsausschuss – FSA).
The FSA is a group of students of a course who take care of the needs of the other students on their course. The FSA, for example, mediates disputes between students and professors, organises parties or collects remembrance exams. The possibilities and the influence of the FSA are strongly dependent on how many helpers are available for its tasks and projects. It is therefore very important that students get involved in the FSA, even if they are not members.
The student committee is only one of the two organs of a student representatives. The second organ is called the student council general assembly (FSVV). It consists of all students on the course and is convened at least once per semester. Important decisions for the future of the student committee are made there. You can read more details in the student committee framework regulations. You can find it here. Do I have a student council?