In previous lessons we learned that when the possessor is a personal pronoun and the possessed a person we can omit the possessive, like in 我朋友. But when we talk about my friend's car, we must use the possessive particle 我朋友的車.
There are no possessive pronouns in Mandarin, just the particle 的, which can be thought of as the English apostrophe-s.
According to collectors of Chinese character frequency statistics, 的 is by far the most frequent character in daily use.