I need to block my opponent's immediate threat. They have X's at positions (1,1) and (1,2), meaning they could win on their next turn by completing the middle row at position (1,0). By placing my O there, I prevent their win. Additionally, this move gives me strategic options - I now have two possible ways to win: either by completing the left column (if I later play at 2,0) or by trying to create a diagonal opportunity from the top-right. Blocking the immediate threat while creating my own winning pathways is the optimal play here.