The Threshold function sets a color channels to 0, if its value is lower than a defined threshold value. It converts it to maximum intensity, if it is greater.

The Threshold value is given by the Bias parameter. Useful values are located between 0 and 100, which represents the range between the lowest and highest possible values for each color channel (0-255).
Setting the Brightness Mode to Normal makes all channels, which are lower than the threshold value, converted to 0. The Bright mode sets a color channel to 255, if it's previous value is greater or equals the threshold value.
You can define, what channels are to be modified by enabling and disabling their checkboxes.