These are small round stones, usually of glass, but sometimes plastic or other materials. They can be markers, counters, game pieces or may fulfill other or multiple roles, depending on the game.
In Mancala, for instance, they are both the moving game pieces and the score counters.
In Zendo, the black and white stones are mostly Marking Stones, though the players also have one of each color to indicate their guess for Mondo. The other color (usually green) are the Guessing Stones: counters of how many guesses each player has accumulated.
Martian Pachinko uses two colors of stones, mostly as markers but one stone is a game piece.