My head hurts understanding this LINQ

static IEnumerable<string> serializeToEnumerable(TreeNode root)
            => root is null 
                ? new string[] { String.Empty } 
                : serializeToEnumerable(root.left)
                        .Concat(serializeToEnumerable(root.right))
                        .Prepend(root.val.ToString());

static IEnumerable<string> serializeToEnumerable(TreeNode root)
            => root is null 
                ? new string[] { String.Empty } 
                : serializeToEnumerable(root.left)
                        .Concat(serializeToEnumerable(root.right))
                        .Prepend(root.val.ToString());