With this slider you can control the volume distribution between left and right speaker. This slider is an overall regulator, which affects the Master Volume. The middle position is the default. Dragging the slider to the left lowers the volume of the right speaker, dragging it to the right vice versa. Of course, these might be swapped if your speakers aren't positioned correctly.

For Surround Systems please be aware that the Master device often regulates only the Front Speakers. This is a limitation of your Soundcard driver.

If your soundcard has no Master device, some other device might get picked by KMix - for most people this is the Wave (or PCM) control.