These pixels are the center points on which Sparkle will perform its sparkling. The user can use the slider to select the threshold value or type in the value in the text input field. If the granularity of the slider doesn't allow the user to select a specific value, the dialog box can resized wider. This will give the slider better resolution. Generally, you'll want a fairly small value for the threshold if you're just trying to simulate a lens flare.
Smaller values make for smaller, thinner flares.
This value is used to determine the intensity of the flare around the spike. Like the Threshold value, the Flare Intensity has both a slider and a text input field for specifying its value. The opacity of the flare determines how much of the flare color will be combined with how much of the original image. If the original image is fairly evenly colored then the flare will look like a normal flare only darker or lighter. Some tests I ran while adding this code produced effects that looked somewhat like paint splashes.