This is the 253rd most frequent Italian word.
nostri
"Our" (plural).
The word 'nostri' is an adjective modifying 'amici' to show possession referring to 'we' or 'us'.
Questi sono i nostri amici.
These are our friends.
In this sentence, 'nostri' describes 'parenti', indicating familial possession associated with 'us'.
Abbiamo trascorso la giornata con i nostri parenti.
We spent the day with our relatives.
Here, 'nostri' specifies to whom the 'books' belong, acting as a possessive adjective.