This is the 376th most frequent Italian word.
vostro
"Vostro" means "your" (plural, formal or informal) in English.
Here, 'vostro' is used as a possessive adjective to indicate that 'the book' belongs to 'you', plural.
Questo è il vostro libro.
This is your book.
In this example, 'vostra' emphasizes possession or responsibility attributed to 'you', plural.
È per colpa vostra che siamo in ritardo.
It is because of your fault that we are late.
In this context, 'vostra' expresses that 'participation' belongs to 'you', plural, stressing involvement.