What is "tuning peg"

A tuning peg is used to hold a string in the pegbox of a stringed instrument. It may be made of ebony, rosewood, boxwood or other material. Some tuning pegs are ornamented with shell, metal, or plastic inlays, beads (pips) or rings.

Turning the peg tightens or loosens the string, changing the pitch produced when the string is played and thereby tuning it.