"Om Mani Padme Hum"
is a mantra used in Tibetan Buddhism. Tibetan Buddhists believe that
saying the mantra (prayer), Om Mani Padme Hum, out loud or silently to
oneself, invokes the powerful benevolent attention and blessings of
Chenrezig, the embodiment of compassion. Viewing the written form of the
mantra is said to have the same effect -- it is often carved into
stones, like the one pictured above, and placed where people can see
them.
Spinning the written form of the mantra around in a Mani
wheel (or prayer wheel) is also believed to give the same benefit as
saying the mantra, and Mani wheels, small hand wheels and large wheels
with millions of copies of the mantra inside, are found everywhere in
the lands influenced by Tibetan Buddhism.
See http://en.wikipedia.org/wiki/Om_mani_padme_hum#Transliterations for more information.