above threshold is a general effects plug-in that can set all pixels that are above a threshold in red, green, and blue to a color you specify.
What should I use it for? This plug is extremely useful whenever you are filming against a bright background (e.g. white) and want to eliminate shadows (for later matting effects) or eliminate slight irregularities in a background. you can also use it to change the color of a bright background to any color you like.
Note that there is a second plug called 'below threshold' that does the opposite: set all pixels that are below a certain level to a color you specify.