I must block position 2,0 (bottom-left) immediately to prevent my opponent from winning on their next move. My opponent already has Os at positions 0,0 and 1,0, forming a vertical threat in the leftmost column. By placing my X at 2,0, I block their win. While I have two Xs in the middle row (1,1 and 1,2), I cannot complete that row because position 1,0 is already occupied by an O. After blocking this immediate threat, I can look for opportunities to create my own winning patterns on subsequent turns.