This is the 93rd most frequent German word.
wo
"Wo" means "where" in English.
In this sentence, 'wo' functions as an interrogative pronoun asking about a location.
Wo bist du jetzt?
Where are you now?
Here, 'wo' acts as a subordinating conjunction introducing a dependent clause about location.
Ich weiß nicht, wo das Buch ist.
I don't know where the book is.
In this example, 'wo' is used colloquially as a relative pronoun meaning 'when' or 'in which' referring to a point in time in the past.