Ultimate Tic Tac Toe also known as Super Tic Tac Toe or Meta Tic Tac Toe, is a board game composed of nine Tic Tac Toe boards arranged in a 3-by-3 grid. Players take turns playing in the smaller Tic Tac Toe boards until one of them wins in the larger Tic Tac Toe board. Strategy in this game is much more conceptually difficult.
Rules:

Each small 3-by-3 Tic Tac Toe board is referred to as a local board, and the larger 3-by-3 board is referred to as the global board.

The game starts with X playing wherever they want in any of the 81 empty spots. This move 'sends' their opponent to its relative location. For example, if X played in the top right square of their local board, then O needs to play next in the local board at the top right of the global board. O can then play in any one of the nine available spots in that local board, each move sending X to a different local board.

If a move is played so that it is to win a local board by the rules of normal Tic Tac Toe, then it wins that local board.

Once the outcome of a local board is decided (win or draw), no more moves may be played in that board. If a player is sent to such a board, then that player may play in any other board.

Game play ends when either a player wins the global board, or there are no legal moves remaining.

For better understanding, you can watch this-
https://youtu.be/9jYcaeSo2f0