In a number of languages Mere possession makes the item that is possessed definite and is reflected by using the. Languages as different as Italian Norwegian and Hebrew do this. Oddly some of the romance languages do this (Romanian Italian Catalan) and others do not (Spanish Portuguese French). For those of us not used to tacking the definite article onto the end of the word it looks rather strange in Romanian. Short answer: if something is possessed it is no longer a something it is the something.
"Vostru" means "your"/ "yours" and it is used for plural. Depending of the sentence, it can be both possessive pronoun (al vostru) or possessive adjective. "Tu" means "you". It is always used for singular, it is personal pronoun. "Voi" means "you" for plural, personal pronoun. "Tău" means "your"/ "yours", is for singular and it can be both possessive pronoun (al tău) or possessive adjective, depending of the sentence.