This is the 3132nd most frequent Russian word.
крупную
"Крупную" means "large" or "big" in English, usually as a feminine accusative form.
In this sentence, 'крупную' modifies 'ошибку' (mistake), indicating its substantial size or significance.
Она сделала крупную ошибку.
She made a big mistake.
Here 'крупную' describes 'рыбу' (fish), showing its notable size.
Мы поймали крупную рыбу.
We caught a big fish.
In this example, 'крупную' complements 'сумму' (sum), emphasizing that the amount of money is large.