The Wohnungsgeber (the "apartment-giver") is the person who gives you a room or an apartment to rent. It can be the landlord, or the main tenant (Hauptmieter). The main tenant must always get the permission of the landlord before subletting a room or subletting the apartment, and the landlord can refuse1, 2.

The Wohnungsgeber can sign the Wohnungsgeberbestätigung. You need this document to register your address in Germany.