Cordons pierreux (lit., 'stone line' in French) are small stone walls. In some cases, not walls, even, but just a row of small stones lined up across fields. They function to stop soil run-off, as when rain pushes silt and biological matter across the ground it will catch behind the cordon. This prevents both the soil from washing away, and also slows the flow of water, giving it more time to soak into the ground. Some cordons pierreux are intended more for wind than rain, and larger walls may build up small dunes behind them.
While these structures are common enough in France -- and hardly unknown elsewhere in the world -- they are currently becoming more common in Africa, as the Brown Revolution encourages better soil management in small and affordable ways.