Moon Rocks, (月の石, lit. Moon Stone), also known simply as Mysterious Cubes, are collectable found in Super Mario Odyssey. They are cubic objects found in Super Mario Odyssey. There is one in each kingdom visited (excluding the Mushroom Kingdom, Dark Side, and Darker Side), but they have no function until after Bowser is defeated in the Moon Kingdom.
Moon Rock are shiny, lustrous, gray cubic objects that are found underneth the Wedding Chapel in the Moon Kingdom.
After defeating Bowser and collecting all of the Multi Moons in a given kingdom (if there are any), that kingdom's Moon Rock will glow, and Mario can activate it either by throwing Cappy at it or Ground Pounding on it. Upon activating a Moon Rock, it will fly into the sky and explode, causing more Power Moons to become available in that kingdom. Toadette also gives Mario a Power Moon for activating all of the Moon Rocks.
Similarly textured rocks can be found in the Moon Kingdom, most prominently during the escape sequence at the end of the game.