I need to block my opponent's potential win. They have X's at positions (0,1) and (1,1), forming a vertical line. If they place an X at (2,1), they'll win the game. By placing my O at (2,1), I block their immediate win threat.

Additionally, this move gives me some future options. I have an O at (0,0), so I could potentially work toward diagonal (with 2,2) or vertical (with 1,0) wins in future moves, depending on how my opponent responds.