Frets are pieces of specially shaped metal wire that get mounted into the neck or fretboard of a stringed instrument. The exact placement of the frets is carefully controlled by a mathematical formula, which is based on the scale length of the instrument (the distance from the nut to the bridge). Fret wire can be made out of a range of materials, but the three most common are 18% Nickel Silver, Stainless Steel and Gold EVO (a proprietary alloy developed for use in eyeglasses).
Fret wire comes in various sizes, ranging from lower/narrower profiles for smaller instruments like mandolins, to larger/wider profiles for bigger instruments like bass guitars.
Frets can be arrayed in various ways to allow different scales to be played on an instrument. The most common is the Chromatic Scale, seen on most modern acoustic and electric guitars, followed by the Diatonic Scale, seen on dulcimers, strumsticks and other more “primitive” instruments.