This is the 46th most frequent French word.
où
"Where."
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.
Je me souviens du jour où nous nous sommes rencontrés.
I remember the day when we met.
Used as an interrogative word to ask about the location of the book.