What is the Java Comparator interface?

import java.util.ArrayList;
import java.util.Collections;
import java.util.Comparator;
public class ComparatorShot {
	
	public static void main(String[] args) {
		ArrayList<Person> arr = new ArrayList<>();
		arr.add(new Person(1, "John"));
		arr.add(new Person(5, "Terry"));
		arr.add(new Person(0, "Lukaku"));
		arr.add(new Person(2, "Pablo"));
		
		// Passing Comparator anonymously:
		Collections.sort(arr, new Comparator<Person>() {
			@Override
			public int compare(Person p1, Person p2) {
				// Sorting by IDs:
				
				if(p1.id < p2.id)
					return -1;
				else if(p1.id > p2.id)
					return 1;
				else
					return 0;
			}
		});
		
		// Displaying sorted array:
		arr.forEach(System.out::println);
	}
}