Nut

The Nut is the part of the guitar where the strings vibrating endpoint lies. The nut is typically made out of plastic, bone, brass, stainless steel, and other various kinds of metal.

The nut also signifies the "zero fret". Also called being "Open". An open chord is a chord that has one of the strings untouched, thus making the final vibrating spot at the nut.

Description
The nut will have grooves cut into the material used. Those grooves are where the strings are placed. The strings will then follow to the tuning machines. There, the strings can be tightened or loosened, allowing the guitar to be tuned.

The nut also needs to be very carefully cut. If not, it can lead to severe tuning problems, such as buzzing, improper tuning, and other minor issues. The nut is located right under the headstock and is on the fret board.