Seeking an explanation of how a "ReadOnlyCollection" works in this given case?

I'm trying to find out what is happening in the following lines and I'm new to the "ReadyOnlyCollection." I was hoping someone could explain it to me. I'm especially confused about the line:

_children = new ReadOnlyCollection<Node>(_intChildren);

public Node Parent { get; }
private List<Node> _intChildren;
private ReadOnlyCollection<Node> _children;
public ReadOnly Collection<Node> Children
{
   get 
   {
      if (_children == null)
         _children = new ReadOnlyCollection<Node>(_intChildren);
      return _children;
   }
}

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