This is the 960th most frequent Hungarian word.
fiú
Boy.
In this sentence, 'fiú' is used as a noun subject meaning 'boy', referring to a young male individual.
A fiú játszik a parkban.
The boy is playing in the park.
Here, 'fiúnak' is the dative case of 'fiú', indicating possession or to whom something belongs.
A fiúnak kék a szeme.
The boy has blue eyes.
In this context, 'fiú' is used in a predicate nominative construction, identifying the subject 'he' as a boy.