This is the 46th most frequent French word.
où
"Where."
Used as an interrogative word to ask about the location of the book.
Où est le livre que j'ai laissé ici ?
Where is the book that I left here?
Used as a relative pronoun to introduce a subordinate clause specifying the location of an action.
C'est un beau quartier où j'aimerais vivre.
It's a nice neighborhood where I would like to live.
Used as a relative pronoun to introduce a subordinate clause specifying the time of an event.