A car is driven around the city on a daily basis will eventually become dirty because of the dirt passing through him. Sometimes it is raining and this is the time, when the road are muddy. It can't be help that a car gets dirty all the way because of these reasons. And that is what just happened. The car asked Pou to clean him up.
The game is played via mouse and the idea here is to make the car look like brand new again. Use the pressurized water to clean up the dirt that has mounted on the car. Brush it well to get rid of those hard to clean tires. Put some soap and soak it fully wet in water.