Java PriorityQueue + class

Hi I am getting an error while trying to use PropertyQueue with java. I am a little unfamiliar with java and would appreciate getting a little bit of explanation with this error! Here is how my code look like roughly:

import java.util.*;
import java.io.*;

class Lecture{
    public Lecture(Strings){
        ...
    }
    //Then some getter and setter and function
}
class Room{
   Vector<Lecture> setLecture = new Vector<Lecture>();
   int end = 1;

   public void change_end(int c){
      end = end +c;
   }

  public void add_lecture(Lecture l){
     setLecture.add(l);
  }

}

public class interval_partioning{

  public static void main (String[] args) {
  // create priority queue
  PriorityQueue <Room>  classrooms = new PriorityQueue <Room> () ; 
  Room classroom1= new Room();
  Room classroom2= new Room();
  classrooms.add(classroom1);
  classrooms.add(classroom2);
 }
}

here is the error I get at this line : classrooms.add(classroom1);

Exception in thread "main" java.lang.ClassCastException: class Room cannot be cast to class java.lang.Comparable (Room is in unnamed module of loader 'app'; java.lang.Comparable is in module java.base of loader 'bootstrap')

What does it mean and how to fix it!

Thank you!