This is a special control for setting frequencies. By clicking on various boxes, you can pick a pitch and octave that translates into a frequency. You can also use the slider to adjust that frequency by +/- 50 cents (a quarter tone). If this is not accurate enough, you can tap on the name of the frequency in the upper left corner, and a normal keyboard will appear.

This control is used to set the Base Key, and the ranges of a String Area.