I need to block my opponent immediately. Looking at the middle row, my opponent (X) already has two symbols at positions 1,0 and 1,1. If I don't block position 1,2, they will win on their next move by completing three Xs in a row. 

Additionally, this move keeps my future winning possibilities open. After blocking, I still have potential winning paths by either completing the right column (if I play at 2,2 next) or by focusing on the bottom row (if I play at 2,2 after blocking).