below threshold (a.k.a. "under threshold") is a general effects plug that can set all pixels that are below 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 dark background (e.g. grey, black) and want to eliminate reflections (for later matting effects) or slight irregularities in a background. you can also use it to change the color of a dark background to any color you like.
Note that there is a second plug called 'above threshold' that does the opposite: set all pixels that are above a certain level to a color you specify.