That skin is a volume controler that show and can change the system volume by clicking or sliding on the bar.
The bar shows only when the mouse is on the skin.
Middle click to switch between vertical and horizontal.
There is a variant that doesn't hide bar.
You can edit the skin variables for the size and the colors.
Works in Windows Vista and higher (not on XP).
Tested on Rainmeter 2.3, but should work as well on 2.2
Feedback and suggestions appreciated
Credits to toastbrotpascal
aka P2C for his slider's code