The Student Parliament (StuPa) consists of 16 elected members and the student senate members, who are also represented in the StuPa.
The StuPa deals with the (financial) budget of the student body (what your semester fee is spent on), personnel issues, as well as important concerns of the student body and the university (e.g. accreditation, parking management and the administration of the TMS).
If you want to know in detail what the StuPa actually does, you are welcome to watch a session. These are public.
In the winter semester, the new StuPa members elect the new members of the AStA (General Student Committee).