An open three is the first real threat on the board: black's row-7 trio at (6,7), (7,7), (8,7) has both ends open, and either end becomes an open four — two five points, one white move, game over. That is why the defender must act NOW, and why the middle block is the classic beginner's error: it touches nothing, leaving both ends alive. Kill it at an end before it grows; which end is a question the rest of this course will keep sharpening.
Block the three at a point that stops the open four.
Finish this step to move on.