Mani stone. The inscription reads "Om mani padme hum", the most common mantra and very prevalent in the context mani stones.
Om Ma Ni Pad Me Hum (six syllables) or Om Mani Padme Hum (four syllables) is also known as the six syllable mantra is the most popular and common Buddhist Mantra carved on rocks and boulders in the Tibetan cultural world.