I need to block X's immediate winning threat. X currently has two marks in the middle column (positions 1,1 and 2,1). If I don't play at 0,1, X will have an opportunity to complete a vertical line and win on their next move. Additionally, playing at 0,1 gives me two Os in the top row, which opens up a potential winning opportunity for me later if I can play at 0,2. This is both a defensive and potentially offensive move.