This is the 73rd most frequent Italian word.
suoi
"Suoi" means "his," "her," or "their" (masculine plural possessive).
In this sentence 'suoi' is a possessive adjective referring to 'friends', indicating possession and matching in number and gender with the noun it modifies.
I suoi amici sono arrivati in ritardo.
His friends arrived late.
Here 'suoi' is a possessive adjective referring to 'books', showing possession by the subject (she) and agreeing with the plural masculine noun.
Lei legge i suoi libri tutte le sere.
She reads her books every evening.
In this case 'suoi' is used as a possessive pronoun linked to 'giocattoli', representing the owner of the toys in conjunction with the context of the sentence.