The house administration (Hausverwaltung) is the people in charge of an apartment building. You call the Hausverwaltung when you have problems in your apartment: loud neighbours, lost keys, problems with utilities etc. In some smaller buildings, this can be the landlord themself. In larger buildings, it’s usually a housing company.
If you don’t know who you Hausverwaltung is, look on your lease. It’s often on a billboard in your apartment lobby.