If you have no German school-leaving certificate (nor a 'Sprachdiplom II' from a 'Deutsche Schule' abroad), you will need an official proof of advanced German language proficiency for admission to a study programme in Germany.
The DSH ('Deutsche Sprachprüfung für den Hochschulzugang ausländischer Studienbewerber') is a test of German for academic purposes and the general requirement for the admission of international students to institutions of higher education in Germany. It tests your language proficiency in terms of your ability to conduct your studies in German.
Alternatively, the TestDaF is also recognized, although HS Mannheim does not offer preparation training for this qualification. Both language exams are offered by official test centres according to a pre-determined schedule. The DSH can only be repeated twice.