The German law clearly defines which German language certificates a student may hand in. However, the universities and each program can make independent decisions when it comes to setting the requirements for the German language level. That is why it is important that you check with the program of your choice to learn precisely which proficiency level they expect from you.
As a rule of thumb, German universities require you to present a proof of upper intermediate to advanced German language skills (level B2/C1) to study in German.
To let applicants know how well they should speak German, many German universities use the terms defined by the Council of Europe in the Common European Framework of Reference for Languages (CEFR). On this scale, there are 6 levels (A1, A2, B1, B2, C1, C2):
A1 is the lowest level and designates that the student is a beginner
C2 is the highest level one can gain
Upper intermediate language skills correspond with level B2, and advanced language skills correspond to level C1.