Barre chords are an integral part of playing guitar. Probably 80% of all chords are barre chords in various forms. A barre chord is formed when you use your index finger to fret all the strings at one fret, then add other fingers to form a chord. This use of the index finger is called a barre. Once you have barred the strings, that fret, in effect, becomes the nut, and any chord you build on top of the barre regards that fret as the nut.