From SKYbrary Wiki
The beta range is used for ground operations inclusive of slowing the aircraft after landing. Below flight idle, the power levers control the blade pitch directly. With the power lever(s) in the ground idle position, the blade pitch is such that the propeller produces its minimum level of thrust. By moving the power lever from the ground idle position towards maximum reverse, the propeller blades go into reverse pitch which directs the airflow from the propeller forward. This will assist in reducing the aircraft speed during the landing roll and also will allow for intentionally moving the aircraft backwards.