I need to block column 5 immediately because my opponent has three Xs in a row horizontally in row 3 (columns 2, 3, and 4). If I don't place my piece in column 5, my opponent will place there on their next turn and win with 4 Xs in a row. Additionally, placing my piece in column 5 adds to my horizontal sequence of Os in the bottom row (columns 4 and 5), which could help setup a future winning opportunity. This is clearly the most critical defensive move on the board right now.