Something that's beaten has been made smooth or thin from repeated impact, like the beaten surface of a silver bracelet, or the beaten path that leads to your hideout in the woods.
To make something beaten, you have to beat it — strike or hammer it repeatedly. Food can also be beaten, like the beaten egg whites you put in a cake to make it light and fluffy. In this case, they're stirred with vigor. And when a person is beaten, they're despondent or defeated: "The beaten expression on his face showed how exhausting the last tennis match had been." Beaten comes from beat and its root meaning "to strike."