The 'a' component is the real part, and the 'b' component is the imaginary part of the complex number. This button will set the number of iterations used to test each point on the screen. A popup window will be displayed asking for an integer value. The number you enter will be used to decide whether each point on the screen belongs in the set. The default value is 100.
Higher values increase the detail, but slow the drawing process down.
After entering the integer, press the OK button. This button will cancel any drawing process and clear the screen. Just hold down the left mouse button and drag the mouse until the rectangle on the screen outlines the area you want to magnify, then let the mouse button up. To deselect an area you have outlined, just click the left mouse button. You may want to increase the number of iterations each time you zoom, in order to increase the detail of the image.