The opponent has three consecutive X pieces in column 3 (at rows 1, 2, and 3) which means that if they drop an X in column 3 on their next move (which would fall to row 0), they would have four in a row vertically and win immediately. To block this immediate threat, I must drop my piece in column 3, which will fill the top cell (row 0) in that column and prevent the win.