I need to block my opponent's vertical threat in column 3. They currently have three 'X' pieces stacked in that column (positions 3,3 - 3,5 from the top-left). If I don't block by placing my 'O' in column 3, they will win on their next turn with a vertical connect-4. While there are other moves that might help build my position (like placing in column 2 to build on my existing piece), none are as urgent as preventing an immediate loss.