An Automatic Sound Leveliser (ASL) automatically adjusts the volume of your car soundsystem to account for changing noise levels inside the car.
When accelerating up to highway speeds the level of ambient noise inside the car usually increases due to the sound of the tyres on the road, wind noise, etc.
The soundsystem detects this increase in ambient noise and increases the volume of your soundsystem accordingly – decreasing it again when you slow down.
The sensitivity of the system can often be adjusted depending on how much or little you want the volume to increase.
ASL systems will likely become a thing of the past soon as more advanced technology such as noise cancelling soundsystems become more common.