PingPongs the value t, so that it is never larger than length and never smaller than 0.
The returned value will move back and forth between 0 and length.
using UnityEngine;public class Example : MonoBehaviour
// Set the x position to loop between 0 and 3
transform.position = new Vector3(Mathf.PingPong(Time.time, 3), transform.position.y, transform.position.z);