Basically on a turbo-prop the propeller is spinning at a constant speed, while in flight, pulling back the power levers to flight idle will flatten out the prop blades and create a lot of drag to slow down, this flattening process still keeps the prop blades at a slightly positive pitch angle, moving power levers forward will cause the blades to take a larger bite for more thrust.
Beta-range is where power lever movements either forward or aft directly control the prop pitch angle through a mechanical linkage, usually accomplished by pulling the power levers backward over a gate mechanism (aft for decelerating on the runway after landing or for backing up). Generally speaking, beta-range is used in ground ops. only (some exceptions). I'm not an engineer as you might be able to tell, just a driver.