This is the Media Volume Slide control. It allows you to control the volume of a laptop, the DVD Player or the VCR.
To raise or lower the volume, place your finger on the media volume slide and drag up or down. You can also place your finger at any point on the media volume slide to make quick volume adjustments.
Press the Mute button to silence the audio from all AV devices (except microphones). To restore the volume, just press the Mute button again.