This is the 177th most frequent Serbian word.
једну
The Serbian word "једну" means "one" (feminine, accusative case).
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.
Од свих песама, отпевао је само једну.
Out of all the songs, he sang only one.
Here 'једну' acts as a numeral ('one') in the accusative case modifying a feminine noun ('књига').