Barre chords essentially are the open string chords moved up the fretboard, with an added twist, the index finger must barre across the fretboard, acting as the open strings to keep the chord shape sounding as it should. The reason why this can be difficult for most beginners, is due to the strength required to hold several frets down with one finger, without causing the strings to buzz or sound muted. Most beginner players have yet to develop this strength, and need to put the time into building this strength through exercise. The following exercises can be used to develop this strength.
Hold the index finger across one fret on the top three strings (G, B and E), use the middle and ring fingers to add extra pressure by placing them on top of the index finger. The goal is to have each string ‘ring’ out clearly with no buzzing or muting. Start out by playing higher up the fretboard where there is less tension, once this becomes comfortable then move down the fretboard towards the first fret. The ring and middle fingers can be removed once the tension starts to feel easier.
This exercise can then be expanded to include more strings gradually, with the end goal being the ability to hold down all six strings across the first fret and have each note ring out clearly.
This strength will take time to build, so a beginner guitarist should practice these exercises regularly whilst respecting the limitations of their fingers, to avoid any unnecessary strain or injury.