resolve the ko
There are basically two ways to resolve a ko (that is, to end a ko):
Black has played a ko-threat to which White decides not to respond :
connects and thereby resolves the ko, in short white connects the ko. (In Japanese, this shape is called 'Umebachi'; name of a flower-shaped Japanese heraldry)
If the marked stone is present, White can resolve the ko in a different way.
It is now more profitable to capture the marked black stone to end the ko. This shows fundamental efficiency of ko; White has ignored atari and secured one point of territory and a likely eye at a by not connecting.