It's all about eye shape
A surrounded group lives or dies on the eye space it encloses, not on how many stones it owns. A space that can still split into two separate eyes means life; a single connected space means death, because the attacker fills it down to one eye and then takes the whole chain. This chapter sorts the standard shapes by point count: a two-point space in the corner is already alive, three points are undecided, four points in a good shape are safe — and every undecided shape has exactly one vital point that both sides want. The black wall here wraps a straight three along the middle row, the classic undecided shape. Black to play: one move in the right place becomes two real eyes.