Opponent currently has three horizontally connected X's in the bottom row (columns 2, 3, and 4). This means if opponent drops an X in column 1 or column 5 on their next turn, they immediately complete a row of four and win. Although blocking one threat won’t stop the opponent from winning on the other column, I have to choose one move. Choosing column 1 will drop my piece to the bottom row, directly blocking one of the winning moves and offering a slim hope for a mistake from opponent later on.