This is the 50th most frequent Russian word.
которая
"That," "which," or "who" (relative pronoun, feminine singular).
Relative pronoun introducing a clause describing the noun 'book'.
Книга, которая лежит на столе, принадлежит мне.
The book that is lying on the table belongs to me.
Relative pronoun connecting the liking to the specific music described in the clause.
Мне нравится музыка, которая играет сейчас.
I like the music that is playing right now.
Relative pronoun specifying which girl is referenced in the context.