what is the z axis in unity code example

Example: x y and z axis in unity

public float playerspeed =5.0f;     // Use this for initialization void Start () {     //Player Spawn Point     transform.position = new Vector3 (0, 3, 0);  }  // Update is called once per frame void Update () {     transform.Translate (Vector3.right * Input.GetAxis("Horizontal") * playerspeed * Time.deltaTime);     transform.Translate (Vector3.forward * Input.GetAxis ("Z") * playerspeed * Time.deltaTime);  }