What does it mean? Trichromat is a collective name for cyan, magenta and yellow (CMY), the three transparent inks that are also known as subtractive primaries. In theory, at least, equal percentages of cyan, magenta and yellow should provide a neutral grey value. In practice the result is a distinctly muddy brown.
The lack of an ideal magenta dye has dogged the history of colour printing. This is just one reason why printers need to use four printing inks, using an adapted black ink to get a satisfactory colour output.
Using black ink to darken areas of an image (undercolour removal or UCR) also allows printers to save money on expensive transparent trichromat inks. It can help to manage certain practical constraints like the number of printing stations and plates that are available on a press to run a job.