This is the 48th most frequent Polish word.
którym
"Którym" means "which" or "whom" (instrumental case).
In this sentence, 'którym' is used as a relative pronoun to refer to the house ('dom'), indicating the location of the action (living).
To jest dom, w którym mieszkam.
This is the house in which I live.
Here, 'którą' (the feminine form of 'którym') is used as a relative pronoun to refer to the feminine noun 'osobę' (person), indicating the relationship 'with whom'.
Zapytaj osobę, z którą rozmawiałeś.
Ask the person with whom you were speaking.
In this example, 'który' (the masculine singular form related to 'którym') functions as a relative pronoun referring to 'dzień' (day), indicating 'that day' I remember.