Propietaria means owner. In this context, it refers to the woman who owns the flat/apartment.
Depending on context, in English you might translate it as:
It is a fairly neutral, standard word.
In Spain, piso commonly means flat or apartment.
This is very common in Peninsular Spanish. A learner may know piso as floor as well, and that is also possible depending on context, but here del piso clearly means of the flat/apartment.
In Spain:
In other varieties of Spanish, apartamento or departamento may be more common.
Because amable is being used as a general characteristic of the person, not a temporary state.
So:
Using está amable would sound unusual here and could suggest she is being kind at the moment, rather than describing her personality.
In this sentence, la is needed because you are talking about a specific person:
Without the article, propietaria would not work naturally here as the subject of the sentence.
Spanish uses definite articles very regularly with nouns, often more than English does.
Yes. Propietaria simply means female owner.
It does not automatically mean someone who rents property out. It just means she owns something. In this sentence, because it says del piso, we understand she is the female owner of the flat, and in context that may be the landlady.
So the exact English translation depends on context: