I need to block my opponent's immediate threat. They have X's at positions (0,1) and (1,1), forming a vertical line. If I don't block position (2,1), they will win on their next move by completing this vertical line.

While I could place my O at (1,0) or (2,0) to work toward my own vertical line with my existing O at (0,0), blocking the opponent's immediate win takes priority. After blocking at (2,1), I can then focus on creating my own winning opportunities in subsequent moves.