It's a pretty basic method of car care. You can google two bucket method and you'll see.
So it's like one bucket has the car wash shampoo solution in it to actually wash the car.
The other bucket has plain water to rinse the wash mitt after it has captured all the dirt from your car. The cyclone dirt trap in the rinse bucket helps keep the dirt from the wash mitt at the bottom of the bucket so you don't scratch your car with the dirt.