It stands for something like "Automatic Sound Leveling."
It adjusts the audio volume in accordance with the vehicle speed. The faster you go, the louder the audio will become to compensate for engine / wind / road noise. It has high, medium, and low levels, and these correspond to how aggressive the volume scaling is. For example, on low, it will only increase the sound a little in proportion to vehicle speed. On high, it will increase it significantly more.
Hope this helps.
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