Reflex and deflex are terms used to explain the geometry of a riser. A deflex riser bow will have a higher brace height than a reflex riser bow which generally- but by no means always- means it is a more forgiving bow.
A reflex riser bow has a shorter brace height which usually translates into a higher performance bow. Reflex/deflex is measured by the position of the throat of the grip in terms of its location from a line drawn from each of the pocket pivot points on the handle.
If the throat of the grip is in front of the pivot point line it is considered deflex, if it is behind the pivot point line it is considered reflex.