Opponent X has an immediate win threat on the bottom row: they already have three consecutive pieces at columns 2, 3, and 4. They can win by dropping an X in either column 1 or column 5. Since I can only block one, I choose to block by dropping my O in column 1, filling the bottom cell (row 5) of that column. This forces the opponent to use column 5 to win next turn, which at least delays the win (though I’m in a losing position).