What this part of code does ?

The script

 using System;
 using UnityEngine;
 using Random = UnityEngine.Random;
 using System.Collections;
 using System.Collections.Generic;
 using System.Linq;
 
 public class CloneObjects : MonoBehaviour
 {
     public GameObject ObjectToCreate;
     public int objectsHeight = 3;
     [HideInInspector]
     public GameObject[] objects;
 
     // for tracking properties change
     private Vector3 _extents;
     private int _objectCount;
     private float _objectSize;
 
     /// <summary>
     ///     How far to place spheres randomly.
     /// </summary>
     public Vector3 Extents;
 
     /// <summary>
     ///     How many spheres wanted.
     /// </summary>
     public int ObjectCount;
     public float ObjectSize;
 
     // Use this for initialization
     void Awake()
     {
         Clone();
         objects = GameObject.FindGameObjectsWithTag("ClonedObject");
     }
 
     private void OnValidate()
     {
         // prevent wrong values to be entered
         Extents = new Vector3(Mathf.Max(0.0f, Extents.x), Mathf.Max(0.0f, Extents.y), Mathf.Max(0.0f, Extents.z));
         ObjectCount = Mathf.Max(0, ObjectCount);
         ObjectSize = Mathf.Max(0.0f, ObjectSize);
     }
 
     private void Reset()
     {
         Extents = new Vector3(250.0f, 20.0f, 250.0f);
         ObjectCount = 100;
         ObjectSize = 20.0f;
     }
 
     // Update is called once per frame
     void Update()
     {
 
     }
 
     private void Clone()
     {
         if (Extents == _extents && ObjectCount == _objectCount && Mathf.Approximately(ObjectSize, _objectSize))
             return;
 
         // cleanup
         var ObjectsToDestroy = GameObject.FindGameObjectsWithTag("ClonedObject");
         foreach (var t in ObjectsToDestroy)
         {
             if (Application.isEditor)
             {
                 DestroyImmediate(t);
             }
             else
             {
                 Destroy(t);
             }
         }
 
         var withTag = GameObject.FindWithTag("Terrain");
         if (withTag == null)
             throw new InvalidOperationException("Terrain not found");
 
         for (var i = 0; i < ObjectCount; i++)
         {
             var o = Instantiate(ObjectToCreate);
             o.tag = "ClonedObject";
             o.transform.SetParent(base.gameObject.transform);
             o.transform.localScale = new Vector3(ObjectSize, ObjectSize, ObjectSize);
 
             // get random position
             var x = Random.Range(-Extents.x, Extents.x);
             var y = Extents.y; // sphere altitude relative to terrain below
             var z = Random.Range(-Extents.z, Extents.z);
 
             // now send a ray down terrain to adjust Y according terrain below
             var height = 10000.0f; // should be higher than highest terrain altitude
             var origin = new Vector3(x, height, z);
             var ray = new Ray(origin, Vector3.down);
             RaycastHit hit;
             var maxDistance = 20000.0f;
             var nameToLayer = LayerMask.NameToLayer("Terrain");
             var layerMask = 1 << nameToLayer;
             if (Physics.Raycast(ray, out hit, maxDistance, layerMask))
             {
                 var distance = hit.distance;
                 y = height - distance + y; // adjust
             }
             else
             {
                 Debug.LogWarning("Terrain not hit, using default height !");
             }
 
             // place !
             o.transform.position = new Vector3(x, y + objectsHeight, z);
         }
         _extents = Extents;
         _objectCount = ObjectCount;
         _objectSize = ObjectSize;
     }
 }
 
 

The part of the script i don't understand:

 var height = 10000.0f; // should be higher than highest terrain altitude
             var origin = new Vector3(x, height, z);
             var ray = new Ray(origin, Vector3.down);
             RaycastHit hit;
             var maxDistance = 20000.0f;
             var nameToLayer = LayerMask.NameToLayer("Terrain");
             var layerMask = 1 << nameToLayer;
             if (Physics.Raycast(ray, out hit, maxDistance, layerMask))
             {
                 var distance = hit.distance;
                 y = height - distance + y; // adjust
             }
             else
             {
                 Debug.LogWarning("Terrain not hit, using default height !");
             }

When i'm running the game using the script it does clone the objects fine. But i keep getting a warning:

"Terrain not hit, using default height !"

I don't understand why i'm getting the warning and what does it mean and how to fix it ? I also don't understand what does the variables height and maxDistance does ?