I need to block my opponent's immediate winning opportunity. They have "O" markers at positions 0,0 and 0,2 (the top corners), so they could win by placing an "O" at 0,1 to complete the top row. By placing my "X" at 0,1, I prevent this win.

Additionally, this move creates a potential winning line for me down the middle column, connecting with my "X" in the center position (1,1). This gives me an attacking opportunity in a subsequent move if I can place at 2,1 to complete the middle column.