Simple mode (the default) hides certain advanced controls for a simpler interface to the program. Advanced mode gives you access to these controls.
If you are new to Fractal eXtreme or like to view fractals mainly for their visually beauty, stick with Simple mode. If you want the most control over the fractals you view in Fractal eXtreme, use Advanced mode.
Advanced mode enables the following:
You select Advanced mode in the Miscellaneous page of the Preferences window.