Explanation of the words of Ibn Malik (may Allah have mercy on him): The Relative Pronoun

The Shaykh : Then the author (may Allah have mercy on him) said: "The Relative Pronoun". The Relative Pronoun is a passive participle. It is called "Relative" because its meaning is not complete except with its relative clause; it is originally broken and needs a connection, or it is cut off and needs a connection. For this reason, it is called "Relative" because its meaning is not complete except with its relative clause. The Relative Pronoun is among the definite nouns—as mentioned before—and what is its rank among the definite nouns?
The student: The fourth.

The Shaykh : The fourth. Yes?