Spinning prayer wheels is a ubiquitous practice that for us has come to typify Tibetan Buddhism.
Prayers and Mantras written on paper fill the insides of the wheels.
Spinning the wheels gathers merit to the pilgrim. It also reflects on the Dharma, the never-ending cycle of life --- birth, life, death, re-birth...
OH MANI PADME HUM is the Mantra that is usually spun in the wheel.