The gradient bar represents the mapping from a pixel's brightness to a new color.
Dark pixels will be assigned to the color on the left of the gradient.
Light pixels will be assigned to the color on the right of the gradient.
The sliders under the gradient bar are known as Markers.
Select a marker by clicking on it. The selected marker will be shown as a white slider with a black outline.
You can move the selected marker's position in the gradient by dragging it.
You can change the selected marker's color with the Set Marker Color button. This will change the colors in the gradient.
Markers can be added or deleted with the Add Marker and Delete Marker buttons.

The Color Blending options let you adjust how the gradient mapped colors blend with the input colors of your image.
The Mix Colors option simply combines the output colors with the input colors.
The Overlay Colors option will overlay the output colors on top of the input colors.
The slider adjusts the overall strength of the effect.
Drag the slider to the left for subtle changes. Drag the slider to the right for more obvious changes.