As the name says, brightness controls how bright your image is. The effect of brightness will also depend on 1/gamma setting (for high values of 1/gamma, brightness will have less impact on the image).
You can either type in values, click on the up and down arrows or click on the arrows and drag.
Brightness effect on a gnarl:
People usually call it just Gamma. Increasing Gamma will basically make your fractal flame more solid.
Low Gamma may create some excessively bright spots.
High Gamma may introduce noise and artifacts (areas that would be almost transparent with lower gamma values will become visible). Also, may make the image look flat by reducing the contrast.
Look how increasing Gamma affects the gnarl:
Now on this image, the Gamma is way too high, highlightng the dark empty areas:
Here, the Gamma is too low, making the central part of the image too bright, and the sides too dark:
While not a perfect setting, this intermediate value of Gamma keeps the sides visible and the center bright, but just enough:
Highlight power (tonemapping)
Highlight power can be used to remove highlights and overstaurated areas. Click the checkbox and set a value.
Highlight power unchecked
Highlight power checked
Higlight power checked and set to 0.1
Be careful though, since high highlight power values can produce an effect opposite of intended, by creating large white areas that will look worse than original overexposure.
Controls your image vibrancy.
Low vibrancy values will result in lighter and less saturated colors.
High vibrancy values will result in darker and more saturated colors.
This setting may be a bit redundant, but very useful when you need to do some quick changes.
Gamma Threshold (tonemapping)
Using this threshold, you can remove the lower gamma areas. Very handy to remove artifacts and noise:
Try not overusing it, because you may end up thresholding too much.
Gamma Threshold can be also used to force a less flat look to high gamma fractals (although it is usually easier to just lower the gamma):
Gamma threshold = 0
Gamma threshold = 1
In Part 2: Curves and background.