This is the 5528th most frequent Russian word.
покупает
Buys.
Here, "покупает" is used in the present tense to indicate that the subject (она) is performing the action of buying milk.
Она покупает молоко в магазине.
She buys milk at the store.
In this sentence, "покупает" describes a habitual action that the subject regularly does.
Каждый раз, когда он посещает город, он покупает сувениры.
Every time he visits the city, he buys souvenirs.
Here, "покупает" emphasizes that he, specifically, is the one performing the action of buying books, distinguishing him from others.