This is the 18317th most frequent Russian word.
шпинату
"Шпинату" is the dative case of "шпинат," meaning "spinach" in English.
In this sentence, 'шпинату' is in the dative case, used after the verb 'добавлять' to indicate to what the spinach is being added.
Я добавил немного шпинату в суп.
I added some spinach to the soup.
Here, 'шпинату' is an incorrect colloquial usage where the accusative case 'шпинат' is expected, reflecting spoken or regional variation.
Ребёнок не хочет есть шпинату.
The child doesn’t want to eat spinach.
In this sentence, 'шпинату' (intended to be 'шпината') could be a rare archaic usage, as in modern Russian the genitive 'шпината' is correct.