Frets are any of the ridges of metal set across the fingerboard of a guitar which help the fingers to stop the strings at the correct points. A fretboard is a fingerboard that is fitted with frets.
The nut of the guitar is the ledge, as of ebony, at the upper end of the fingerboard, over which the strings pass.
We number the frets in relation to their distance from the nut.
A note can be played either by playing the string “open” at the nut, or you can use your finger(s) to fret a note(s).
To fret a note you place your finger on the fretboard behind the actual fret. The string will come into contact with the metal fret to sound the correct note.