The Quality setting determines how well defined the fractal image is in lower density areas. Low density areas exhibit random noise in fractal images that are rendered with low quality (below 20 or so).

Quality defines the number of iterations per pixel that the flame fractal rendering algorithm uses to generate the rendered image. So quality is directly proportional to rendering time. A quality value of 1000 will take 20 times longer to render the same image as quality 50. That is because 20 X 50 = 1000.

So for fast renders, use low quality settings. For excellent final image rendered quality, use a quality setting.

Fractal Architect uses different quality settings in different parts of the application. Time and speed critical areas use low quality. The preview window uses higher quality (but no where near what a good final rendered image for exhibition would use.)

Fractal Architect Quality Preferences

Preview window - 55
Editor Thumbnails - 25
Thumbnail Batches - 15
Render to File - 300

As a rule of thumb, large images need higher quality values, as the noise in low density areas is much easier to see. With small thumbnails, you cant see the noise very well anyway, so lower quality suffices.