Pointer question

Hi folks,

I am confused by pointer, pointer to pointer. I do not understand code below (why  binTreeNode ** root?)

Could you pls give me your most personal & favotite view on pointer, pointer to pointer? Thanks. Pls based on code below. However, comparable & similar codes, if applicable, using pointer, pointer to do great job is appreciated. Many thanks.

Have a nice day!

Inserting a New Node in the Tree


void insert(int newKey, binTreeNode ** root)
// Pre: root points to the root of the tree or NULL
// Post: insert a new node in the subtree of lesser height
{
int leftHeight, rightHeight;
if (*root == NULL)
*root = createNode(newKey, NULL, NULL);
else
{
leftHeight = height((*root)->leftChild);
rightHeight = height((*root)->rightChild);
if (leftHeight < rightHeight)
insert(newKey, &((*root)->leftChild));
else
insert(newKey, &((*root)->rightChild));
}
}