This is the 57th most frequent Czech word.
ji
"Ji" in Czech means "her" or "it" (accusative form of "ona" – she).
The word 'ji' is used here as an accusative form of 'ona' (she) referring to a person being observed.
Vidím ji v parku.
I see her in the park.
In this sentence, 'ji' functions as an object pronoun in the accusative case referring to the person who is singing.
Počkejte, až ji uslyšíte zpívat.
Wait until you hear her sing.
Here 'ji' is the accusative pronoun for 'kniha' (book), showing the action of reading directed towards the book.