This is the 4966th most frequent Japanese word.
空腹
"Empty stomach; hunger."
The word '空腹' (pronounced 'kuufuku') is used here to describe the state of being hungry in a general way, typifying a physical sensation of emptiness in the stomach.
空腹の時は何か軽いものを食べると良いです。
When you're hungry, it's good to eat something light.
In this sentence, '空腹' is used causatively to explain the reason behind an action—the sensation of hunger prompted him to act.
彼は空腹のため、晩ご飯を早く作った。
Because he was hungry, he made dinner quickly.
Here, '空腹' is employed descriptively to predict or state a common occurrence—hunger following physical activity.