The filter selects parts of the image at which pixels have the similar color. Threshold value parameter indicates the main color which would be searched at the image. Transformation type indicates the method of search of the nearest color (RGB – in the RGB space, Hue — according to hue parameter in HSV model). Radius indicates maximum space between the main color and color of pixel by which the pixel shall be selected.
In threshold transformation you can select one of the Operations (treshOperation):
1) binary transformation (treshBinary). Pixel whose value is at the distance not more than Radius from the main color, becomes white, lower — black.
2) inverse binary transformation (treshBinaryInv). Pixel, whose value is at a distance not more than Radius from the main color becomes black, lower — white.
Please find more information on settings and the use of filter in the step by step instruction Tutorial on the Threshold of color image filter.