What is a fret on a guitar and how does it affect the sound produced by the instrument?

A fret on a guitar is a raised metal strip along the neck of the instrument. When a guitarist presses down on a fret while playing a string, it shortens the vibrating length of the string, which changes the pitch of the note produced. The placement of the fret determines the pitch of the note, and pressing down on different frets allows the guitarist to play different notes and create melodies.