This is the 275th most frequent Dutch word.
waarop
"Waarop" means "on which" or "upon which" in English.
In this sentence, 'waarop' is used to indicate a point in time (the day) when an event (everything changed) happened.
Dit is de dag waarop alles veranderde.
This is the day on which everything changed.
Here, 'waarop' refers to an abstract reference, indicating a book and connecting it to the action of basing a decision.
Ik zoek een boek waarop ik mijn keuze kan baseren.
I am looking for a book on which I can base my decision.
In this example, 'waarop' involves a spatial relationship, where the table is described as the surface bearing the glass.