This window allows the user to modify parameters specified in the AFL formula via Param, ParamStr and ParamColor functions.
It is accessible via chart context menu (right click the mouse over the chart pane to see the context menu) : choose the 4th item down, "Parameters" and a small window with a slider will appear. By highlighting the item from the list on the left side of the window you select given parameter. Then depending on type of the parameter (number, string, color) appropriate controls will appear.
If given parameter is a number then slider and the edit field will be shown as in the picture below
You can move the slider using mouse, <- -> cursor keys and mouse wheel. As changes are made underlying chart is immediately refreshed giving great feedback for the user.
If given parameter is a string then text field will appear, and if given parameter is color then color-picker control will allow you to change the color.
For more information on using parameters please read Tutorial: Using colors, styles, titles and parameters in Indicator Builder