I followed the video exactly and checked my code, everything works except the set tool tip text piece:

main class:
import javax.swing.JFrame;

public class GUI2 {

   public static void main(String[] args) {

      GUISUB1 sub = new GUISUB1();
      sub.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
      sub.setSize(275, 180);
      sub.setVisible(true);
   }

}
Sub Class:
import java.awt.FlowLayout;
import javax.swing.JFrame;
import javax.swing.JLabel;

public class GUISUB1 extends JFrame{

   private JLabel item1;

   public GUISUB1(){
      super("The Title bar");
      setLayout(new FlowLayout());

      item1 = new JLabel("This is a Sentence");
      item1.setToolTipText("This is gona show up on hover");
      add(item1);
   }
}