Its a fret in the "zero" position, i.e. when playing an open string its resting on a fret. If you want to see them you can find a pic of an MTD...
do you actually have to fret the note or is it always there EDIT: whats the use of that ( looked it up)
Some people say it makes open notes sound more like their fretted cousins. I personally don't think it's that big of a deal, but to others it might be.
My Steinbergers have zero nuts. It's nice not having to fit a nut when I radically change types of string.