I need to play in column 5 to block my opponent's immediate vertical win threat. They currently have three X's in a row vertically in column 5 (positions 5,3 through 5,5). By placing my piece in column 5, it will fall to the top available position and prevent their potential winning move next turn. This is a defensive move that addresses the most urgent threat on the board, as I don't currently have any immediate winning opportunities myself.