This is the 197th most frequent Japanese word.
どこ
"Where"
In this sentence, 'どこ' (doko) asks about a location where the meeting will be held.
次の会議はどこで開催されますか？
Where will the next meeting take place?
Here, 'どこ' is used to inquire about the location of the passport in a situational context.
パスポートがどこにあるかわかりません。
I don't know where my passport is.
In this instance, 'どこ' is used to ask which aspect or section of the store is popular.