A LUT is basically a table that says if the input is 1 make the output 7. If the input is 0 make the output 5 – or whatever, these numbers are arbitrary, it is just a conversion matrix – take this, make it that. This can happen to the image over all or per colour channel (RGB). For example in the red channel use LUT A to make 1 = 7 but in the blue channel use LUT B to make 1=15.