A machine head is the part of a
guitar used for winding the strings and holding them at a specific tension.
It consists of a knob which, when rotated, turns a peg perpendicular to it which goes into the head of the guitar. The guitar string end is placed through a hole in this peg and, as the knob is turned, the string is wound to the proper tension for whatever note the string is to be tuned to. After re-tuning new strings to the right note a few times, they will be effectively 'broken in' and retain their tension.