If you want to move gameobjects back and forth in unity than you can do this by using a function  Mathf.PingPong. This function takes two float
values we can say 'q' and 'length'. It will ping-pong the value 'q' so that it can never be larger than 'length' and never smaller than zero.

To do this create a 3D gameobject by going to Menu>Gameobject>3D Object>Cube and place it at position(0,0,0). After this create a C# script and name it as CubeMovement and attach it to the gameobject you created.

Add the following code to the script:

using UnityEngine;
using System.Collections;
 
public class CubeMovement  : MonoBehaviour
{
  public float speed = 2;
  void Update()
{
  transform.position = new Vector3(Mathf.PingPong(Time.time*speed, 4), transform.position.y, transform.position.z);
}
 
}

This will move the cube from (0, 0, 0) to (4, 0, 0) and back to (0, 0, 0) and so on. You can apply this to other axis as well.