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.

	function Update () {
		// Set the x position to loop between 0 and 3
		transform.position = Vector3(
				     Mathf.PingPong(Time.time, 3), transform.position.y, transform.position.z);
	}

using UnityEngine;
using System.Collections;

public class Example : MonoBehaviour {
    void Update() {
        transform.position = new Vector3(Mathf.PingPong(Time.time, 3), transform.position.y, transform.position.z);
    }
}

import UnityEngine
import System.Collections

public class Example(MonoBehaviour):

	def Update() as void:
		transform.position = Vector3(Mathf.PingPong(Time.time, 3), transform.position.y, transform.position.z)