I must play in column 5 to block my opponent's immediate winning threat. They have three X's in a row horizontally in row 4 (columns 2-4), and would win next turn if they play in column 5. My piece will occupy the bottom position in column 5, forcing their next piece to land in row 4 if they choose that column. This move also begins to build my own potential opportunities on the right side of the board.