When you move into an apartment, you get a tenancy confirmation (Wohnungsgeberbestätigung). This document proves that you live there. You need it to register your address, or to apply for a residence permit.
You get the Wohnungsgeberbestätigung from your landlord. If you live in a sublet or a WG, you can also get it from the main tenant. They need the landlord’s permission first.
The Wohnungsgeberbestätigung is also called Einzugsbestätigung des Wohnungsgebers or Vermieterbescheinigung. It’s sometimes the same document as the Übergabeprotokoll.