Horizontal inversion (mirror). I was equally confused though the first time I was asked lol.

I would have called it "reversing", but I guess that sounds less technical. Now I'm confused, just in a different way. There's no obscure algorithm for this, just maybe 4 straight-forward lines, depending on language. I guess it tests whether you've ever heard of recursion.

I thought the meme depended on it being some obscure, difficult, and irrelevant thing.

Well, now you know. It's a meme that has become its own meta-meme.

The root is still the root, but everything else is just flipped: https://leetcode.com/problems/invert-binary-tree/

It is also extremely simple:

  TreeNode* invertTree(TreeNode* root) {
    if (root) {
      invertTree(root->left);
      invertTree(root->right);
      std::swap(root->left, root->right);
    }
    return root;
  }