what does ienumerator mean unity code example

Example: unity ienumerator

using System.Collections;
using System.Collections.Generic;
using UnityEngine;

public class ExampleScript : MonoBehaviour
{

	void Start()
    {
    	StartCoroutine("example");
        //Start Coroutine
    }

	IEnumerator example()
    {
    	//Code in IEnumerator
    }
}