I must block column 6 immediately. My opponent has three X pieces in a row at positions 3, 4, and 5 on the bottom row. If they place a piece in column 6, they will win with 4 in a row horizontally. By placing my O in column 6, I prevent their immediate win while also starting to build my own position. Column 2 already has my piece, so the horizontal threat is only at column 6.