This is the 599th most frequent Polish word.
właściciel
Owner.
Here, 'właściciel' is in the nominative case and describes a person who owns the apartment.
Nowy właściciel mieszkania przeprowadził remont.
The new owner of the apartment has conducted a renovation.
The word 'właścicielem' appears in the instrumental case to define the person responsible for the shop.
Musimy porozmawiać z właścicielem tego sklepu.
We need to talk to the owner of this shop.
In this sentence, 'właścicielem' is used in the instrumental case to indicate possession by the subject.