This is the 838th most frequent Romanian word.
căreia
"Whom" or "to whom" (feminine singular, genitive/dative).
The word "căreia" is used as a relative pronoun, relating to the female noun "prietena", and acts as an indirect object with a possessive nuance.
Aceasta este prietena căreia i-am spus secretul meu.
This is the friend to whom I told my secret.
Here, "căreia" functions as a relative pronoun referring to "persoană", which is indicated as the recipient of the action in the subordinate clause.
Am întâlnit o persoană căreia i-ai scris o scrisoare anul trecut.
I met a person to whom you wrote a letter last year.
In this sentence, "căreia" is a feminine relative pronoun specifying "profesoara" as the beneficiary of the gratitude expressed.