This is the 280th most frequent Croatian word.
kući
"Kući" means "home" (to the home).
In this sentence, 'kući' serves as an adverb describing the state 'at home'.
On je kući cijelo popodne.
He is at home the entire afternoon.
In this case, 'kući' is an adverb that specifies a destination, 'to home'.
Želim se vratiti kući.
I want to return home.
Here, 'kući' is used as an adverb to indicate the direction 'to home'.