A GUI click is normally press-and-release, so 0.5 click would be just-press. Faster!

(Also it's a joke)

Well, sometimes I push the mouse button down on the wrong place, realize it, move the mouse away and release the button elsewhere ... usually that half-click does nothing ;-)

My mouse makes 2 click sounds actually, once on the down press and once on the release. We've all been double clicking this whole time!