how does addall work in java code example

Example: ArrayList addAll(int index, Collection c) method in java

import java.util.ArrayList;
public class ArrayListAddAllMethod
{
   public static void main(String[] args)
   {
      // creating an empty ArrayList with initial capacity
      ArrayList<Integer> al1 = new ArrayList<Integer>(5);
      al1.add(11);
      al1.add(13);
      al1.add(15);
      // printing all elements of al1
      System.out.println("ArrayList 1 elements: ");
      for(Integer number : al1)
      {
         System.out.println("Number: " + number);
      }
      // creating another empty ArrayList with initial capacity
      ArrayList<Integer> al2 = new ArrayList<Integer>(5);
      al2.add(12);
      al2.add(14);
      al2.add(16);
      al2.add(18);
      // printing all elements of ArrayList2
      System.out.println("ArrayList 2 elements: ");
      for(Integer number : al2)
      {
         System.out.println("Number: " + number);
      }
      // insert all elements of al2 at third position
      al1.addAll(2, al2);
      System.out.println("Print all elements: ");
      // printing all elements in al1
      for(Integer number : al1)
      {
         System.out.println("Number: " + number);
      }
   }
}