After , black is alive with 8 points of territory. (It looks like 10 points right now, but later, as the outside liberties are filled in, black will need to capture the two stones.) White can't kill: if white at a then black plays b.
Black needs to play at to make sure of the second eye. This way, black only has 5 points of territory.
If black plays , then white can kill the corner. Black only has false eyes at and at .