Les adverbes relatifs where, when & why peuvent être utilisés pour relier des phrases ou des propositions. Ils remplacent la structure plus formelle composée d'une préposition + which dans une proposition relative.

Structure formelle, préposition + which Structure plus commune avec un adverbe relatif
That's the restaurant in which we met for the first time. That's the restaurant where we met for the first time.
That picture was taken in the park at which I used to play. That picture was taken in the park where I used to play.
I remember the day on which we first met. I remember the day when we first met.
There was a very hot summer the year in which he was born. There was a very hot summer the year when he was born.
Tell me the reason for which you came home late. Tell me (the reason) why you came home late.
Do you want to know the reason for which he is angry with Sally? Do you want to know (the reason) why he is angry with Sally?