Answer:
As previous person answering said, it is for "Engine Braking" - when going downhill, it is the equivalent to selecting a low gear to remove excess energy rather than pressing the brakes.
As with all other positions of the gear selector on the Prius (except 'P'), there is no mechanical change in the transmission, and it is just an input to the computers - it differs from 'D' mode only when you release your foot from the gas, at speeds over 10 MPH. Then it provides about 3 times the deceleration force compared to 'D' mode without pressing the brake pedal, and will provide this from some combination of an increased regeneration current and/or forcing the engine to turn at high speed without injecting fuel.
Unless the hill is long enough that the battery will be full before you reach the bottom, you're better off applying gentle pressure on the brake pedal, as the physical brake pads don't engage unless the battery is full or you press HARD.
Back-up?
It's the trans brake. Meaning that when you let off of the accelerator the vehicle will slow slightly (regenerative braking) rather than "free wheeling". Any braking above 10mph is actually the generator slowing the vehicle and charging the HV batteries. A regular automatic transmission does this all of the time unless you manually shift to neutral.
A better description can be found in the owner's manual.
It activates engine breaking, for going down long hills. It is much like down-shifting in other cars. It actually does not activate the regenerative breaking, though, so people watching their MPG usually reserve B for very long hills.
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