This is the 12833rd most frequent Russian word.
купишь
"Купишь" means "you will buy."
'Купишь' in this sentence indicates a future action regarding buying, aligning with an implicit dependency (upon buying bread).
Когда купишь хлеб, позвони мне.
When you buy bread, call me.
Here, 'купишь' serves again as a future tense verb specifying the action of buying within a question format.
Ты точно купишь эту книгу?
Are you sure you will buy this book?
The word 'купишь' here is used in the future tense, second person singular (ты) form of 'купить', meaning 'you will buy'.