You’ll have to enable the mode in the options screen before you can use it.
It makes casting spells much more precise, since you don’t have to aim in a bind – instead pausing the combat and taking your time. When fighting large enemies, you can aim for certain body parts, instead of the enemy as a whole. As the developers say: “With Wait Mode, any player can fight like a pro!”.
Final Fantasy XV is going to launch on September 30th, on Playstation 4 and Xbox One. There’s no word on a possible PC port yet, but hopefully that will change as the release date nears.