[adapted from Wikipedia] An ovoo is a type of shamanistic cairn found in Mongolia, usually made from rocks or from wood. Ovoos are often found at the top of mountains and in high places, like mountain passes. They serve mainly as religious sites, used in worship of the mountains and the sky as well as in Buddhist ceremonies, but often are also landmarks.
When I traveled, we often stopped at the ovoo and walked around it 3 times clockwise for good luck. Usually we would add a rock or candy or a little money to the ovoo as an offering.