I need to block my opponent's diagonal win threat. X has three pieces diagonally at positions (1,3), (2,4), and (3,5). If X plays at (4,6), they would win immediately. By playing in column 6, I place my piece at (5,6), preventing this diagonal win. Additionally, this move starts setting up my own potential win, as I already have three O's horizontally in row 4 - if I can later place a piece at (4,6), I would win.