This is the 6429th most frequent Korean word.
어디
"어디" means "where."
In this sentence, '어디' is used as a question word to inquire about a location someone wants to go to.
어디에 가고 싶어요?
Where do you want to go?
Here, '어디' is part of '어디에서', which specifies the location of an action (buying in this case).
어디에서 이 책을 샀어요?
Where did you buy this book?
In this usage, '어디' appears within '어딘가' (meaning 'somewhere'), which gives an indefinite reference to a location.