This priority option is unlocked by purchasing it for 1000MM, no NK coins. When you click on a bloon after selecting "click", the selected tower will target the bloon/blimp/other thing and keep attacking it unless you de-select or it is destroyed. There's also a hot-key in which you can click to automatically set all tower's priority to click. It overrides all other priorities, but cannot override a Distraction bloon. It is useful for defeating the Goodbye Bloon.