TestDaF is a centralised and standardised examination for German as a foreign language. It serves applicants to prove their knowledge of German for university studies. TestDaF is recognised both internationally and by all universities and colleges in Germany as a proof of language proficiency. German universities generally require TestDaF level 4 (TDN 4) in all four skills for admission. In some cases, universities also deviate from this - this should be requested in good time from the individual universities.
The language application requirements (for studying) at TU Braunschweig can be viewed here: Application requirements
TestDaF tests at level B2/C1, so that as a student applicant you should have reached a completed B2 level before registering for this test.