It's a small (125x20) frame that displays your current cash like in the backpack frame. It is dragable and you can show and hide it via chat commands. Also via commands you can change the alpha of the background and the border. The cash amounts and coin icons are clickable to pickup money the same as per the backpack cash display.

"/quickcash" (or "/qc") brings up a list of commands which are:

"/qc show" - toggle on
"/qc hide" - toggle QC off
"/qc lock" - prevent mouse dragging
"/qc unlock" - allow mose dragging again
"/qc alpha number" - set the opacity of the background
"/qc borderalpha number" - set the opacity of the border
"/qc stick" - instructs QC to follow the mouse until it is clicked or unstick is typed.
"/qc unstick" - releases QC from the mouse.
(NB stick and unstick are for moving QC when it has been obscured by another frame. If you don't understand what this means then you probably don't need to worry about the commands.)

The alpha number can be the following values:
0 is fully transparent
1-255 for varying opacities. 1 is solid, 255 is fully transparent.

I assume it works fine in the other language versions of WoW, except that the text for the instructions will remain english.