This is the 5th most frequent Slovenian word.
ki
"Ki" in Slovenian means "who," "which," or "that" (relative pronoun).
The word 'ki' is used as a relative pronoun introducing a clause that provides more information about the noun 'knjiga' (book).
To je knjiga, ki sem jo včeraj prebral.
This is the book that I read yesterday.
Here, 'ki' is also a relative pronoun introducing a clause describing the subject 'nekoga' (someone).
Poznam nekoga, ki ti lahko pomaga.
I know someone who can help you.
In this sentence, 'ki' is a relative pronoun relating to 'mesto' (city) to introduce the relative clause 'ki mi je všeč' (that I like).