This is the 177th most frequent Serbian word.
једну
The Serbian word "једну" means "one" (feminine, accusative case).
Here 'једну' acts as a numeral ('one') in the accusative case modifying a feminine noun ('књига').
Имам једну књигу.
I have one book.
In this sentence, 'једну' emphasizes a single entity ('one story'), again in the accusative case.
Испричаћу ти једну причу.
I will tell you one story.
'Једну' here specifies one item ('one song') from a group, in the accusative case as it's the object of the action in the sentence.