Karl Knechtel: See also almost almost fill.

But actually, the order is wrong: the circled move ought to come second, with the first being the stone beside it - just play two stones across the middle. Intuitive: block the middle points, across which Black seeks to divide the space. Then Black is already dead.

As shown, White has wasted two plays, and presumably offered Black chances to make seki or live outright (depending on what the exact order was):

W can only make seki (at a), if these are the first two plays  

Black simply lives  

Black *already* alive  

White simply kills