Driving a car, much less owning one, is a big deal in Japan. Student driving is a big business in Japan and anyone who has ever navigated Japan's back roads can attest to the skill that the drivers must have to drive on the narrow roads that wind in and around the cities and towns. The roads are narrow and it takes courtesy for the other cars that share the road to let the other car pass. It is not uncommon to park the car and help the other car pass by stopping traffic or guiding them around a tight turn.
This sticker tells other drivers that the driver of this car is a novice or less experienced, so please be patient if they are not as skilled or fast as other drivers. Please be patient.