This is the 796th most frequent Russian word.
купить
To buy.
The verb 'купить' is used here to indicate a desire or intention to acquire an item through purchase.
Я хочу купить книгу.
I want to buy a book.
'Купить' is used in the past tense 'купила' to describe the action of purchasing completed by the subject in the past.
Моя приятельница купила новую машину.
My friend bought a new car.
In this sentence, 'купить' is used in the infinitive form to express the necessity of making a purchase in the future.