This is the 18th most frequent Czech word.
která
"Which" or "that" (feminine singular form).
In this sentence, 'která' is a relative pronoun introducing a relative clause that provides additional information about 'přítelkyni' (girlfriend).
Mám přítelkyni, která miluje cestování.
I have a girlfriend who loves traveling.
'Která' here works as an interrogative pronoun, used in a question to specify 'kniha' (book).
Která kniha se ti nejvíc líbí?
Which book do you like the most?
In this sentence, 'která' (that) is again a relative pronoun but is used here to describe the noun 'klíče' (keys), indicating which keys were placed here.